Remove --pre from tox install command

To avoid installing pre-release versions of packages.

Change-Id: Ia8429d7936a2c0720e342edd733d3c5537c1f564

tox.ini

@@ -1,4 +1,5 @@
 [tox]
+minversion = 1.6
 envlist = py26,py27,pep8
 
 [testenv]
@@ -6,6 +7,7 @@ setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C
+install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =