Fix Travis-CI

* Travis-CI doesn't have a configured signatory, so don't sign for SNAPSHOTS
Merge pull request #25 from BoltsFramework/grantland.signing

Fix Travis-CI + Signing
Use AndroidExecutors on Android

* Fallback to java.util.concurrent.Executors on JVM
* Publicize bolts.android.AndroidExecutors
* Create BoltsExecutors