Update to newest gradle version, needed for Android Studio 0.5.0+.

Won't work with the newest Android Studio (0.5.0+) otherwise:

```
Failed to refresh Gradle project 'TextSecure'
         The project is using an unsupported version of the Android Gradle plug-in (0.8.3).
         Version 0.9.0 introduced incompatible changes in the build language.
         Please read the migration guide to learn how to update your project.
         Open migration guide, fix plug-in version and re-import project
(http://tools.android.com/tech-docs/new-build-system/migrating_to_09)
```