Pip 1.2 compatibility

I don't fully understand the system pip uses, but it appears that in versions later than 1.2, calling pip.version_control() is unnecessary, because that functionality is included by default.

Here is the relevant pip commit where version_control was removed: https://github.com/pypa/pip/commit/56dc8360f45a8e3890f3eca01aca88ee97b3e667