Updates tox.ini to use new features

tox 1.6 allows us to skip the sdist step, which is slow. This does that.
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: Idcc32f7191df85eaac35f4f07256847b677b27fe

tox.ini

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