Update Gradle Android plugin to 0.8+

Otherwise build fails, at least on my machine:
```
   1
FAILURE: Build failed with an exception.

* Where:
Build file '/Users/moe/Workspaces/android/simpletask-android/build.gradle' line: 12

* What went wrong:
A problem occurred evaluating root project 'simpletask-android'.
> Could not create plugin of type 'AppPlugin'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --debug option to get more log output.

BUILD FAILED
```