Override tox's desire to install pre-releases

Define an installation command in tox.ini to avoid using the --pre
argument to pip, which installs a bad version of Sphinx that doesn't
work with pbr.

Change-Id: I09b6ab97cc197cd74857cf6ee4a34a2694523411

Author: dhellmann

tox.ini

@@ -2,6 +2,7 @@
 envlist = py27,py26,py33,zsh,ksh,style
 
 [testenv]
+install_command = pip install -U {opts} {packages}
 commands = bash ./tests/run_tests {envdir} []
 deps = virtualenv
        virtualenv-clone