Fix up build.gradle dependencies for newer Android Studio

I don't think this tiny change should affect earlier versions or IntelliJ either.

    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!