Make JVM builds aware of gradle wrapper

JVM alreads know about build.gradle to make builds using pre-installed
gradle binaries. But Gradleware recomends a feature[1] where a gradle
wrapper script can be used so build can be more smart about required
gradle versions.

This commit makes Travis smart about Gradle smartness.

[1]: http://www.gradle.org/docs/current/userguide/gradle_wrapper.html
Maintain the same code style for checking file existence
Merge pull request #126 from andrewhr/ar-gradle-wrapper-for-jvm

Make JVM builds aware of gradle wrapper