Add dependency on empty version of listenablefuture. (#58)
Add dependency on empty version of listenablefuture. (#58)

This avoids the classpath conflict when both guava and listenablefuture
libraries are present.

See https://groups.google.com/forum/#!topic/guava-announce/Km82fZG68Sw
for context.
firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk