Don't install pre-release software with tox

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>
Merge "Don't install pre-release software with tox"