Fix override_settings compat

[carltongibson]

Closes #2466 Ref #5658

As per #2466, the override_settings integration was updating the APISettings object correctly but was not functioning as expected since the documented usage from rest_framework.settings import api_settings is bound at import time.

This PR adds a (failing) test checking compat and adjusts the code to refresh APISettings in place, rather than replace the instance. override_settings then works as expected.

Fix was suggested by @daggaz — thanks for that! (PRs always welcome! 🙂)

[carltongibson]

@rpkilby If you have a moment, can I ask you to look at this quickly? There's just a small query over the Travis build for the first commit.

It correctly fails, but only for the Python 2 builds. It should have failed for all except lint, readme, docs.

Running tox locally on 3f9a54f I get the expected result:

ERROR:   py27-django110: commands failed
ERROR:   py34-django110: commands failed
ERROR:   py35-django110: commands failed
ERROR:   py27-django111: commands failed
ERROR:   py34-django111: commands failed
ERROR:   py35-django111: commands failed
ERROR:   py36-django111: commands failed
ERROR:   py34-django20: commands failed
ERROR:   py35-django20: commands failed
ERROR:   py36-django20: commands failed
ERROR:   py35-djangomaster: commands failed
ERROR:   py36-djangomaster: commands failed
  lint: commands succeeded
  docs: commands succeeded
  readme: commands succeeded

As such, I'm happy, but I'm just "What happened to Travis?".

I'm going to merge this as is, since I want to finish #5658. I don't think there is a problem; if there is we can come back to it; I'd be grateful though if you could double check.

Ta. 🙂