Fix pip call in tox config

Tox passes --pre to pip, which means we get pre-release
versions of stuff. That breaks the docs build, but is
just a bad idea anyway, so override the setting so we
don't do it.

Change-Id: Iec89b12a969f34486005e8dc41cb189438b38959

tox.ini

@@ -2,6 +2,8 @@
 envlist = py26,py27,py32,py33,style,docs
 
 [testenv]
+# tox passes --pre to pip by default, but we don't want that
+install_command = pip install {opts} {packages}
 distribute = False
 commands = nosetests -d --with-coverage --cover-inclusive --cover-package stevedore []
 deps =