travis: Run tests on trusty instead of precise
travis: Run tests on trusty instead of precise

Travis' APT system is quite unmaintained, so they haven't processed my
request to use wheezy packages from apt.mopidy.com on precise, and are
still using "stable" which is now jessie.

kingosticks/mopidy-spotify,mopidy/mopidy-spotify,jodal/mopidy-spotify