Fix "./gradlew check" task dependencies

[walles]

Before this change, just doing ./gradlew check would fail because FindBugs required assembleDebug to run first.

This change tells Gradle about this dependency and enables one to do ./gradlew check without doing assembleDebug first.

[walles]

Note that this change leaves .travis.yml untouched, because even if we can do check now without assembleDebug we still want to ensure we can actually build / assemble everything.

[goober]

Thanks.