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 were not
asking for.

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

Change-Id: Id751f7d5d40a80d348a5f5936539d9462b40b14b

tox.ini

@@ -1,7 +1,10 @@
 [tox]
 envlist = py26,py27,py33,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