Update 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: Ief0a44a8d982a12ff5158c1223487d27b75b950e

tox.ini

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