Update plugin to work with 0.11 of Android plugin

The plugin was using a `runtimeJar` property of the Android `BasePlugin` that
is no longer available in version 0.11 of that plugin. Instead, we should use
the `bootClasspath` property, as described by [this blog comment][1].

[1]: http://melix.github.io/blog/2014/06/grooid.html#comment-1423846638