Travis CI: upgrade build OS to Ubuntu Trusty

[dhimmel]

Should improve build time on Travis. As per https://docs.travis-ci.com/user/trusty-ci-environment/:

The advantage of running with sudo: false is dramatically reduced time between commit push to GitHub and the start of a job on Travis CI, which works especially well when one’s average job duration is under 3 minutes.

[coveralls]

Coverage remained the same at 90.625% when pulling 698fdd2 on dhimmel:travis into 019512c on suminb:master.

[dhimmel]

Looks like the new Travis containers don't support pypy yet. @suminb if pypy tests are important to you then you can close this PR.

[coveralls]

Coverage remained the same at 90.625% when pulling b1499ee on dhimmel:travis into 019512c on suminb:master.

[suminb]

I suppose testing for pypy isn't super important at the moment, but it would be nice if you could comment out the line (- pypy) and provide some additional comment explaining why you're commenting it out, instead of completely deleting it. That way, when Travis guys fully support pypy with Ubuntu Trusty we can revive that single line. Otherwise I will highly unlikely remember to do so ;-)

[dhimmel]

it would be nice if you could comment out the line

Done. I didn't squash the commits assuming you can use the "Squash merge" feature on github.

[coveralls]

Coverage remained the same at 90.625% when pulling fffd2f6 on dhimmel:travis into 019512c on suminb:master.

[coveralls]

Coverage remained the same at 90.625% when pulling fffd2f6 on dhimmel:travis into 019512c on suminb:master.