Update Android Plugin and Gradle Versions
- Updated Android Plugin from 0.12 to 0.13
- Updated Build Tools Version from 20.0.0 to 21.1.1
- Updated Gradle version from 1.12 to 2.1
Merge pull request #2 from Vungle/gradle_to_2.1_android_plugin_to_0.13

Update Android Plugin and Gradle Versions