Add testing for Django 1.11 and 2.0

[jeromelebleu]

Apart from adding testing for Django 1.11 & 2.0 and fixing tests, it also:

• move test requirements to a separate file tests/requirements.txt for clarity
• update package classifiers for pip
• update Django minimum version in install_requires according to tested versions
• remove six from install_requires as it only rely on Django compatibility utils.

It also drops support for Django 1.7 as it does not pass tests anymore, there is an issue with pytest that I didn't investigate as this version is just too old...

[coveralls]

Coverage remained the same at 92.585% when pulling 4748349528d6b54e1921fbbdab40f0fef20c1fc5 on jeromelebleu:tests/add-1.11-2.0 into 5dcdb1b6c2e23185900002ff8b847a606e6fd911 on fusionbox:master.