Add support for Django 1.11

[dukebody]

• Update middleware to work with Django 1.11 and older versions.
• Add Django 1.11 to the Travis test matrix.
• Update README to remove mention to Django 1.11 not being supported.
• Avoid repeated patching using a middleware class-level flag instead of checking function names.

Resolves #78.

[coveralls]

Coverage decreased (-0.2%) to 95.817% when pulling 052cb1a on dukebody:django-1.11 into 69db1cf on kencochrane:master.

[kencochrane]

@dukebody thank you for adding the Django 1.11 support. It looks like Travis failed because 1.11 doesn't support python 2.6 and 3.3. I think you just need to update the Travis config file so that it doesn't run the tests for those python versions. We do that for other versions as well, so it should be straight forward. If not let me know.

[dukebody]

@kencochrane Done!

[coveralls]

Coverage decreased (-0.2%) to 95.817% when pulling 39a2d2b on dukebody:django-1.11 into 11ee7bc on kencochrane:master.

[kencochrane]

@dukebody thank you! Looks better. going to merge now.