Я разрабатываю приложение для Android с использованием RxJava и Retofit. Когда я тестирую свой код в API 23, я не получаю ошибок, однако, когда я тестирую в API 15, мой RxJava не работает должным образом, и мое приложение вылетает с ошибкой:

FATAL EXCEPTION: main
java.lang.ExceptionInInitializerError
at $Proxy1.createToken(Native Method)
at com.loginsignup.presenter.LoginPresenterImpl.tryToLogin(LoginPresenterImpl.java:39)
at com.loginsignup.view.LoginActivity.tryToLogin(LoginActivity.java:197)
at com.loginsignup.view.LoginActivity.onClick(LoginActivity.java:180)
at android.view.View.performClick(View.java:4084)
at android.view.View$PerformClick.run(View.java:16966)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NoClassDefFoundError: rx.plugins.RxJavaPlugins
at rx.Observable.<clinit>(Observable.java:63)
at $Proxy1.createToken(Native Method) 
at com.gooroo.gooroo.loginsignup.presenter.LoginPresenterImpl.tryToLogin(LoginPresenterImpl.java:39) 
at com.gooroo.gooroo.loginsignup.view.LoginActivity.tryToLogin(LoginActivity.java:197) 
at com.gooroo.gooroo.loginsignup.view.LoginActivity.onClick(LoginActivity.java:180) 
at android.view.View.performClick(View.java:4084) 
at android.view.View$PerformClick.run(View.java:16966) 
at android.os.Handler.handleCallback(Handler.java:615) 
at android.os.Handler.dispatchMessage(Handler.java:92) 
at android.os.Looper.loop(Looper.java:137) 
at android.app.ActivityThread.main(ActivityThread.,java:4745) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:511) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
at dalvik.system.NativeStart.main(Native Method)

Вот моя сборка Gradle:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"

    defaultConfig {
        applicationId "com.example.ex"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "0.1 alpha"
        multiDexEnabled true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    dexOptions {
        javaMaxHeapSize "4g"
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    testCompile 'junit:junit:4.12'
    debugCompile 'com.squareup.leakcanary:leakcanary-android:1.4-beta2'
    releaseCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.4-beta2'
    testCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.4-beta2'
    compile files('libs/sinch-android-rtc-3.9.5.jar')
    compile 'com.android.support:appcompat-v7:23.4.0'
    compile 'com.android.support:design:23.4.0'
    compile 'com.android.support:support-v4:23.4.0'
    compile 'com.android.support:cardview-v7:23.4.0'
    compile 'com.google.android.gms:play-services-maps:8.4.0'
    compile 'com.google.android.gms:play-services-location:8.4.0'
    compile 'com.google.code.gson:gson:2.6.2'
    compile 'com.google.android.gms:play-services-analytics:8.4.0'
    compile 'com.squareup.picasso:picasso:2.5.2'
    compile 'com.squareup.okhttp3:okhttp-urlconnection:3.2.0'
    compile 'com.squareup.okhttp3:okhttp:3.2.0'
    compile 'com.squareup.okhttp3:logging-interceptor:3.2.0'
    compile 'com.squareup.retrofit2:retrofit:2.0.0'
    compile 'com.squareup.retrofit2:converter-gson:2.0.0'
    compile 'com.squareup.retrofit2:adapter-rxjava:2.0.0'
    compile 'com.github.orhanobut:logger:1.12'
    compile 'com.viewpagerindicator:library:2.4.1@aar'
    compile 'io.reactivex:rxandroid:1.2.0'
    compile 'io.reactivex:rxjava:1.1.5'
    compile 'com.facebook.android:facebook-android-sdk:4.+'
    compile 'com.stripe:stripe-android:+'
    compile 'com.google.android.gms:play-services-gcm:8.4.0'
    compile 'com.timehop.stickyheadersrecyclerview:library:0.4.3@aar'
    compile 'com.amazonaws:aws-android-sdk-core:2.+'
    compile 'com.amazonaws:aws-android-sdk-s3:2.+'
    compile 'com.google.android.gms:play-services-wallet:8.4.0'
    compile 'com.android.support:multidex:1.0.0'
}

apply plugin: 'com.google.gms.google-services'

Любая идея, почему нет проблем с API 23, но когда я запускаю его на API 16, он падает, когда я пытаюсь войти в систему, которая использует RxJava?