Log Message:
Update py-virtualenv to 15.0.0.

15.0.0 (2016-03-05)

    Remove the virtualenv-N.N script from the package; this can no longer be \ 
correctly created from a wheel installation. Resolves #851, #692
    Remove accidental runtime dependency on pip by extracting certificate in the \ 
subprocess.
    Upgrade setuptools 20.2.2.
    Upgrade pip to 8.1.0.