remove --use-mirrors from pip commands, it's deprecated.

travis complains that `--use-mirrors` is deprecated, and it's actually already removed in current pip versions. it seems like the mirrors got replaced by a CDN, so it seems that it's enough to just remove `--use-mirrors`.

see http://pip.readthedocs.org/en/latest/news.html and https://pypi.python.org/mirrors

@02strich, could you confirm this?