Don't have tox install pre-release software

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: I3aa2a69c5bae54932137c78cc711236a0ca24602
Merge "Don't have tox install pre-release software"