Portanto, estou executando o ProGuard pela primeira vez e estou recebendo um erro estranho:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:proguardRelease'.
> java.io.IOException: Can't read [C:\Users\Some\workspace\MyApp\app\bin\classes] (No such file or directory)

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

Aqui está o meu gradle:

apply plugin: 'android'

android {
    compileSdkVersion 19
    buildToolsVersion '19.1.0'

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion 19
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            runProguard true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
}

dependencies {
    compile 'com.android.support:support-v4:19.0.+'
    compile 'com.google.android.gms:play-services:4.3.23'
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.viewpagerindicator:library:2.4.1@aar'
    compile group:'com.android.support', name:'appcompat-v7', version:'18.0.+'
    compile 'com.squareup.picasso:picasso:2.3+'
    compile 'com.github.castorflex.smoothprogressbar:library:0.4.0'
}

allprojects {
    gradle.projectsEvaluated {
        tasks.withType(JavaCompile) {
            options.compilerArgs << "-Xlint:unchecked" << "-Xlint:deprecation"
        }
    }
}

meu proguard-rules.txt:

-injars      bin/classes
-injars      libs
-outjars     bin/classes-processed.jar
-libraryjars /usr/local/java/android-sdk/platforms/android-9/android.jar

-dontpreverify
-repackageclasses ''
-allowaccessmodification
-optimizations !code/simplification/arithmetic
-keepattributes *Annotation*

-keep public class * extends android.app.Activity
-keep public class * extends android.app.Application
-keep public class * extends android.app.Service
-keep public class * extends android.content.BroadcastReceiver
-keep public class * extends android.content.ContentProvider

-keep public class * extends android.view.View {
    public <init>(android.content.Context);
    public <init>(android.content.Context, android.util.AttributeSet);
    public <init>(android.content.Context, android.util.AttributeSet, int);
    public void set*(...);
}

-keepclasseswithmembers class * {
    public <init>(android.content.Context, android.util.AttributeSet);
}

-keepclasseswithmembers class * {
    public <init>(android.content.Context, android.util.AttributeSet, int);
}

-keepclassmembers class * extends android.content.Context {
   public void *(android.view.View);
   public void *(android.view.MenuItem);
}

-keepclassmembers class * implements android.os.Parcelable {
    static ** CREATOR;
}

-keepclassmembers class **.R$* {
    public static <fields>;
}

-keepclassmembers class * {
    @android.webkit.JavascriptInterface <methods>;
}