AURORA-227: Aurora build should check for the Python version

Testing Done:
Running ./gradlew checkPythonVersion with Python != 2.6 or 2.7 results in:

:checkPythonVersion FAILED

FAILURE: Build failed with an exception.

* Where:
Build file '/home/daniel/Projects/incubator-aurora/build.gradle' line: 257

* What went wrong:
Execution failed for task ':checkPythonVersion'.
> Build requires Python 2.6 or Python 2.7

Bugs closed: AURORA-227

Reviewed at https://reviews.apache.org/r/18537/