Fixed overzealous initialization of PYTHONPATH.

This is a bugfix over __init__.py that was inititalliy changed at
https://github.com/reviewboard/reviewboard/commit/915452d483a812ee34006741a6f5c23a4d03a5b1

The current initialization is overzealous and conflicts with the
installation call from extensionbrowser's easy_install usage (with the
current initialization, an easy_install call would nuke all entries in
the easy_install.pth file and overwrite previous entries).

Reviewed at http://reviews.reviewboard.org/r/4027/
Merge branch 'release-1.7.x'