Travis builds are failing with because of --use-mirrors

[ppetr]

Excerpt from such a build:

...
0.39s$ if [[ $TRAVIS_PYTHON_VERSION == '2.6' ]]; then pip install --use-mirrors unittest2; fi
Usage:   
  pip install [options] <requirement specifier> [package-index-options] ...
  pip install [options] -r <requirements file> [package-index-options] ...
  pip install [options] [-e] <vcs project url> ...
  pip install [options] [-e] <local project path> ...
  pip install [options] <archive url/path> ...
no such option: --use-mirrors
...

Apparently --use-mirrors is deprecated. Probably the easiest way would be to remove the option, but I'm not that familiar with the underlying reasons.

Question Canonical replacement for --use-mirrors on SO might also be helpful.