travis: Add OpenJDK 8 to build matrix
travis: Add OpenJDK 8 to build matrix

Closes #34

RxBroadcast/RxBroadcast,whymarrh/RxBroadcast,RxBroadcast/RxBroadcast,whymarrh/RxBroadcast,RxBroadcast/RxBroadcast,RxBroadcast/RxBroadcast