Force virtualenv pip to be used

[Alec Taylor]

Running Ubuntu 16.10 with Python 2.7.12+ (default one) and virtualenv
15.0.3 (`sudo -H pip install virtualenv`). What am I doing wrong?

    $ virtualenv a && . "$_"/bin/activate && pip --version
    New python executable in /tmp/a/bin/python
    Installing setuptools, pip, wheel...done.
    pip 9.0.0 from /usr/local/lib/python2.7/dist-packages (python 2.7)
    $ /tmp/a/bin/pip --version
    pip 9.0.0 from /usr/local/lib/python2.7/dist-packages (python 2.7)
    $ /tmp/a/bin/python -c 'from pip import __file__; print __file__'
    /usr/local/lib/python2.7/dist-packages/pip/__init__.pyc

PS: Running with regular Bash 4.3.46(1)-release in GNOME Terminal. When I
activate the virtualenv this appears in my env output: VIRTUAL_ENV=/tmp/a

PPS: Cross-posted [yesterday] http://stackoverflow.com/q/40438089