setup-virtualenv attempts to install the wrong version of the distro package

So currently the setup-virtualenv script relies on setup.py to install all of the required pip packages required by the marionette tests.

I don't know why it does this, but as a result the script attempts to install the distro version 1.7 package which is incompatible with python2.7. Manually installing distro using pip before running setup.py will resolve/install the correct version 1.6 package.