Log Message:
Update to 1.9.1:

1.9.1 (2013-03-08)
~~~~~~~~~~~~~~~~~~

* Updated to pip 1.3.1 that fixed a major backward incompatible change of
  parsing URLs to externally hosted packages that got accidentily included
  in pip 1.3.

1.9 (2013-03-07)
~~~~~~~~~~~~~~~~

* Unset VIRTUAL_ENV environment variable in deactivate.bat (Pull #364)
* Upgraded distribute to 0.6.34.
* Added ``--no-setuptools`` and ``--no-pip`` options (Pull #336).
* Fixed Issue #373. virtualenv-1.8.4 was failing in cygwin (Pull #382).
* Fixed Issue #378. virtualenv is now "multiarch" aware on \ 
debian/ubuntu (Pull #379).
* Fixed issue with readline module path on pypy and OSX (Pull #374).
* Made 64bit detection compatible with Python 2.5 (Pull #393).