Update requirements.txt to disallow alpha libs

This is an update that has been submitted to the
main openstack-requirements file.

The stable/icehouse branch specified an alpha build,
this patch removes that.

Requirement patch can be found here:
https://review.openstack.org/#/c/103674/

Further background can be found here:
http://lists.openstack.org/pipermail/openstack-dev/2014-June/thread.html#39001

Change-Id: I3471ff0fc5f82476ae712e29733c7ae2e3701904