Use Gradle task to install pre-commit hook

[msridhar]

Right now we hack gradlew to install our formatting pre-commit hook. #294 remove this in the process of doing a Gradle update. But I think that's good; it was hackish anyway. We should use a Gradle task to do the installation instead. WALA's build.gradle shows how to do it:

https://github.com/wala/WALA/blob/96408b265b2b431f24b221104cfdb89be1c59fd8/build.gradle#L284-L295

We can then make some common Gradle task dependent on installGitHooks to install the hook automatically.

[lazaroclapp]

100% agreed, but let's not remove it from .gradlew until we have this set up, just for the sake of keeping master green even on new git clones.