Updates 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: Ib51796afbfdffb5a879046ca1b94a55b1f6ef6f1

tox.ini

@@ -1,8 +1,12 @@
 [tox]
 envlist = py26,py27,pypy,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
 commands = python setup.py testr --slowest --testr-args='{posargs}'