Remove upper bound on six dependency version

The upper bound on the six dependency version conflicts with the
lower bound on the transitive six dependency pulled in via the
latest 0.4.1 keystoneclient.

(We also increase the lower bound on the WSME dependency version
to avoid a potential conflict between six 1.4.x and WSME 0.5b2)

Change-Id: I4f00ff756e80397b6c3829b482c3b074b47824df