Don't double "remove" an item from "dirs"

Use "elif" to only call `dirs.remove(dir)` once if both conditions are true. This is possible if a virtualenv called "test" is present, so both "test/bin/python" exists, and `dir == 'test'`. It would execute `dirs.remove('test')` twice, the second time raising an exception.
Merge remote-tracking branch 'matt/patch-1' into develop

Conflicts:
	pip/req.py
Merge pull request #1230 from mgood/patch-1

Don't double "remove" an item from "dirs"