for which you can use
pip install -I or pip install --ignore-installed which will Install from collected packages but if you always want to install a new in your virtualenv you can use the -U flag instead of -I which will first download the latest one and the install it.
Source: http://stackoverflow.com/questions/12079607/make-virtualenv-inheri
Related: http://bitprophet.org/blog/2012/06/07/on-vendorizing/