He integrado Crashlytics en mi proyecto, ahora cuando habilito proguard, App Crashes. He intentado excluir todas las clases de crashlytics sin éxito

El archivo proguard-rules.pro tiene lo siguiente

-keep class io.fabric.sdk.android.** { *; }
-keep interface io.fabric.sdk.android.** { *; }
-keep class com.digits.sdk.android.core.** { *; }
-keep interface com.digits.sdk.android.core.** { *; }
-keep class com.twitter.sdk.android.** { *; }
-keep interface com.twitter.sdk.android.** { *; }

-keep class io.fabric.** { *; }
-keep interface io.fabric.** { *; }
-keep class com.digits.** { *; }
-keep interface com.digits.** { *; }
-keep class com.twitter.** { *; }
-keep interface com.twitter.** { *; }
-keep class com.crashlytics.** { *; }
-keep interface com.crashlytics.** { *; }
-keep class com.crashlytics.** { *; }
-dontwarn com.crashlytics.**

Recibo un error como

com.example.app E/Answers: Failed to submit events task
    java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@1d060ef0 rejected from java.util.concurrent.ScheduledThreadPoolExecutor@31c1ac69[Shutting down, pool size = 1, active threads = 0, queued tasks = 1, completed tasks = 9]
        at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2011)
        at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:793)
        at java.util.concurrent.ScheduledThreadPoolExecutor.delayedExecute(ScheduledThreadPoolExecutor.java:298)
        at java.util.concurrent.ScheduledThreadPoolExecutor.schedule(ScheduledThreadPoolExecutor.java:503)
        at java.util.concurrent.ScheduledThreadPoolExecutor.submit(ScheduledThreadPoolExecutor.java:602)
        at java.util.concurrent.Executors$DelegatedExecutorService.submit(Executors.java:593)
        at com.crashlytics.android.answers.AnswersEventsHandler.executeAsync(Unknown Source)
        at com.crashlytics.android.answers.AnswersEventsHandler.processEvent(Unknown Source)

También miré el documento crashlytics y probé todas las combinaciones posibles. alguien puede guiarme en esto