Update tox.ini to use new features

Per this email message from Monty:

http://lists.openstack.org/pipermail/openstack-dev/2013-September/015495.html

We need to duplicate the changes from this review:

https://review.openstack.org/47848

Change-Id: I82b3cc16e554d21fbb642629f8d021c8b1e50b87

tox.ini

@@ -1,7 +1,11 @@
 [tox]
 envlist = py27,pep8
+minversion = 1.6
+skipsdist = True
 
 [testenv]
+usedevelop = True
+install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'