This improves our documentation for the gradle protobuf plugin, as its
version is dependent on the gradle version.
Gradle now has the --tests flag, performance improvements, and support
for OpenPGP subkeys.
com.android.tools.build:gradle:1.1.0 doesn't work with Gradle 2.10, but
1.5.0 does.
I also bumped the protobuf-gradle-plugin to be the same as the version
used in the README and our primary build.gradle.