Only require unittest2 on Python < 2.7.

This helps with Python 3 compatibility since unittest2 fails to install
on Python 3.
.travis.yml: Add "install" step that does `pip install --use-mirrors .`
so that package installs are more likely to succeed.
Merge pull request #108 from msabramo/python3/unittest2_py27_or_less

Only require unittest2 on Python < 2.7.
Merge pull request #110 from msabramo/travis_pip_use_mirrors

.travis.yml: Add "install" step that does `pip install --use-mirrors .`