Upgrade Gradle and the Android plugin

Currently, the most mature Android Studio version is 0.8.6 beta. The beta
version requires at least version 0.12.0 of the Android plugin for Gradle,
so use that. Also upgrade to the latest supported Gradle release 1.12 in
the same run.
Merge pull request #43 from sschuberth/gradle-upgrade

Upgrade Gradle and the Android plugin