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: I2af4dc6ab460fd8a368b22e3ffbde9467c23f774
Merge "Have tox install via setup.py develop"