Run Android tests with Gradle

Summary:
Make sure that the gradle tests pass before publishing the snapshot. Also makes
sure that we don't break the gradle build.

Reviewed By: emilsjolander

Differential Revision: D5020044

fbshipit-source-id: dfebd829701717eac3ed078a807cd10f489f4348

.travis.yml

@@ -114,6 +114,7 @@ script:
   - |
     if [[ $TARGET = "android" ]]; then
       buck build --verbose 0 //android/sample:sample &&
+      ./gradlew testDebugUnit &&
       scripts/publish-snapshot.sh
     fi