Fix Travis build with latest Mopidy stable

[palfrey]

As you can see from such fun items as https://travis-ci.org/mopidy/mopidy-local-sqlite/jobs/112828386 the version of Ubuntu Travis uses by default (Precise) is so dated it can't install Mopidy stable. This PR swaps over to the newer version (Trusty), which fixes the build issue.

[tkem]

Since mopidy-local-sqlite and its unit tests don't use any of mopidy's non-Python dependencies, just removing the addons section from .travis.yml should also do the trick.

[tkem]

Looks good to me. Could you just git rebase --interactive develop to squash this all into a single commit? Otherwise, that's quite a history for a relatively minor thing...

[palfrey]

Done

[tkem]

Thank you very much!