Use Ubuntu/trusty for testing

[tk0miya]

In 2017 Apr, Ubuntu/precise will reach its EOL.
Hence, the testing environment for master branch should be moved to next LTS; Ubuntu/trusty.
Fortunately, travis released trusty container as a beta feature.
https://blog.travis-ci.com/2016-11-08-trusty-container-public-beta/

This also allows to test LaTeX builder on TeXLive 2013 (refs: #3070)

[tk0miya]

Hmm... testing with pypy is always failed... it seems a bug of travis...

pypy is not installed; attempting download
Downloading archive: https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/14.04/x86_64/-.tar.bz2
$ sudo tar xjf -.tar.bz2 --directory /
bzip2: (stdin) is not a bzip2 file.
tar: Child returned status 2
tar: Error is not recoverable: exiting now
The command "sudo tar xjf -.tar.bz2 --directory /" failed and exited with 2 during .
https://travis-ci.org/sphinx-doc/sphinx/jobs/197384350

[tk0miya]

Finally I could not find the way to avoid the error.
So I just posted an email to the support list.
I hope it will be fixed soon...

[tk0miya]

I give the version to env spec to avoid the pypy problem.
refs: travis-ci/travis-ci#6865

It seems good.

[tk0miya]

Merged.
From now on, master branch is tested on Ubuntu/trusty (14.04)!