refactor(client): use semver for --version
feat(client): check server version

If the client sends a X-Deis-Version header to the server, a django
middleware class will validate and ensure that the versions are the
same. If they are not, the request is rejected, and a HttpResponse
object is returned with the client and server versions displayed.
fix(controller): reverse version output

Thanks johanneswuerbach for catching this!
test(controller): add django middleware tests
Merge pull request #1165 from deis/554-client-version

feat(client): check client version against the server