(1) Add py35 job to Travis.
(2) To address issue #57: run tests that connect to Mojang's auth server exactly once, during the py35 job.
(3) Measure coverage exactly once, during the py35 job; always submit the result to coveralls.
(4) Fix miscellaneous errors in generate_travis_yml.py.
- Return value of _make_request() is treated as a requests.Request, when it is in fact a requests.Response.
- Some tests in test_authentication use assertRaises() incorrectly, resulting in testing code that never gets run.
- Other miscellaneous errors exposed by the above changes.
Additionally:
- YggdrasilError instances now have fields with specific error information, and _raise_from_response() populates them. (This will be useful for later changes.)
