Updates tox.ini to use new features

Reasons:
 - tox update v1.6

Changes:
 - tox 1.6 allows us to skip the sdist step, which is slow.
 - It also allows us to override the install line. In this case, it's
   important as it allows us to stop getting pre-release software we
   weren't asking for.

Original patch by Monty Taylor, talked about here:
http://lists.openstack.org/pipermail/openstack-dev/2013-September/015495.html

Change-Id: I7ea029bc75547ccd43566d3329037757d0388112

tox.ini

@@ -1,7 +1,11 @@
 [tox]
 envlist = py26,py27,pep8
+minversion = 1.6
+skipsdist = True
 
 [testenv]
+usedevelop = True
+install_command = pip install {opts} {packages}
 setenv = LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C