Updated from global requirements

Change-Id: I071601a07d74711828dc961b646dc066262698da
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: Iddf85d33b57f256d72756b516c23492e9600bedc