Use pip instead of easy_install for installation.

If we're running the installation of dependencies for installation
ourselves anyway - just use pip and be done with it. This means
that our requirements will get installed consistently regardless of
whether we're using pip, python setup.py install or a tox environment.

Change-Id: If92557a33a76553ad36bd136fa87780857a894b1
Merge "Use pip instead of easy_install for installation."