Have tox install via setup.py develop

tox 1.6 was released, which means that we can now take advantage of
the feature we added to it - which is using setup.py develop to
install the code into the virtualenv. The logic was taken from
run_tests.sh - so the performance issues around using tox vs. using
install_venv should now be gone.

Additionally, override the tox pip install command to avoid using the
"--pre" option which is the default in tox.  "--pre" means "Include
pre-release and development versions."  By default, pip will only
install stable versions of software, and that is the behavior we want.

Change-Id: Ida5e440d1bdb9f8e9031277ea53a02d2ef171438
Co-Authored-By: Monty Taylor <mordred@inaugust.com>