Update gradle plugin version

This went crazy with 1.x upgrade to Android Studio, apparently
for no reason. I don't know why we should specify it more
carefully than this.

The actual error was
The project is using an unsupported version of the Android Gradle plug-in
which is really hard to track down to just a classpath in the
build.gradle!
Merge pull request #144 from rfay/20141204_new_gradle

Fix up build.gradle dependencies for newer Android Studio

Pulling this myself since I don't think it has any impact and probably doesn't need review. My apologies in advance if I was wrong!