travis: Use trusty and pygobject instead of pgi

Created by: SunilMohanAdapa

• Run tests on Ubuntu Trusty (14.04) instead of Ubuntu Precise (12.04).
• Use pygobject instead of pgi to test what we actually use in production.
• Use a virtualenv trick to make sure that Python actually picks up system packages (python3-gi). https://pygobject.readthedocs.org/en/latest/testing.html#example-travis-ci-configuration
• Don't let apt-get hang for user input. Specify -y.