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>

tox.ini

@@ -5,6 +5,7 @@ envlist = py26,py27,py33,pep8
 
 [testenv]
 usedevelop = True
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en