pip module wasn't adding -e when using VCS
Remove use_mirrors from pip module

PyPI moved to a CDN on 2013-05-26, so mirrors are now largely
unnecessary and pip removed the functionality in 1.5. More importantly
(and why I wrote this request): on 2014-02-15 the mirror directory was
taken offline, so mirrors may not work anymore even for pip versions
that support them.
Merge pull request #6045 from garaden/pypi_mirrors_gone

Set use_mirrors default to "no" for the pip module