Use robovm plugin 0.0.8, resolve #1813

This is a pre-emptive PR and should not be merged until the gradle RoboVM plugin is on central: http://search.maven.org/#search|gav|1|g%3A%22com.github.jtakakura%22%20AND%20a%3A%22gradle-robovm-plugin%22

* Updates to use the new RoboVM gradle plugin that uses RoboVM 0.0.12
* Allows you to have an empty text field in the android sdk text box if you don't have the android module selected