auth: correct URL used to get auth token

I must somehow have been testing using a locally modified version of
the file. Without '/tokens' at the end of the URL, you get this error
back from Keystone:

  {"error":
    {"message": "get_version_v2() got an unexpected keyword argument 'auth'",
     "code": 400,
     "title": "Bad Request"}}
Merge pull request #47 from mgeisler/fix-auth-token-url

auth: correct URL used to get auth token