Update Readme.md

Readme.md

 Heroku buildpack: Python
 ========================
 
-This is a [Heroku buildpack](http://devcenter.heroku.com/articles/buildpacks) for Python apps.
-It uses [virtualenv](http://www.virtualenv.org/) and [pip](http://www.pip-installer.org/).
+This is a [Heroku buildpack](http://devcenter.heroku.com/articles/buildpacks) for Python apps, powered by [pip](http://www.pip-installer.org/).
 
 [![Build Status](https://secure.travis-ci.org/heroku/heroku-buildpack-python.png?branch=master)](http://travis-ci.org/heroku/heroku-buildpack-python)