travis integration tests failing due to end-of-lifed Precise platform

[warner]

Recently, our pull requests have been failing CI, because Travis-CI is using the "Precise" platform (Ubuntu 12.04 LTS), which reached its end-of-life back in april. It appears that torproject.org no longer offers .deb packages for precise, so the "integration" tests (which try to install the latest Tor via a torproject.org PPA) fail.

The fix will be to update our .travis.yml to request a newer platform, probably Xenial (16.04 LTS).

[warner]

Oh, it turns out that the integration test script (integration/install-tor.sh) explicitly added a precise-based URL to the APT sources.list. So that needs to be changed to match.

[Brian Warner <warner@…>]

In 14c0379/trunk:

travis: switch from EOLed 'precise' to 'trusty'

closes ticket:2888

[warner]

I went with trusty (14.04 LTS) instead of xenial (16.04 LTS) because we've already got a xenial buildbot worker, so I wanted coverage of the oldest supported Ubuntu flavor.