iPhone 6/6+ launch images, Gradle 2.1 and dependency version updates.

**NOTE:**
* **Requires robovm plugin version -alpha-02 (Not released yet)**
* **Broken for IDEA users! They need to use IDEA 14 pre-release or wait till the official release. (due fall)**




iOS:
* Add the launch images for iPhone6 and iPhone6+ so apps can run at the native resolution.
* Change the artifact ID now it is under the robovm namespace.

Android:
* Default android version of 20
* Default build tools version of 20.0.0

Gradle:
* Using Gradle 2.1 for the wrapper.

Misc:
* Fix build tool version comparison.


Related issues: #2415 , #2407