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: I87f172cfcc44c9afff9c5162791d6c6b31972702

tox.ini

@@ -1,8 +1,12 @@
 [tox]
 envlist = py26,py27,py33,pep8
+minversion = 1.6
+skipsdist = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
+usedevelop = True
+install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        setuptools_git>=0.4