Allow webob 1.3

Both Trusty and Sid have WebOb 1.3, so we need to allow
WebOb 1.3.

Change-Id: I1c64cf4ff659b6a2ef0d6915bf83f9eb5e44a8e9
Merge commit 'refs/changes/81/68181/1' of git://gold.eng.hq.xensource.com/git/github/openstack/requirements into tmpbranch