Pin Django version for nosedjango compatibility

Until nosedjango works with Django 1.6, this is the simplest fix.
Django 1.6 requires a CACHE key in settings

This is just future proofing – if tests are to ever run with Django 1.6,
this needs to be present.
Only process response if the request reached htmlmin
Extend RequestMock, add RequestBareMock w/o flag

RequestMock always pretends that htmlmin has seen the request, so
all other tests work normally.
Add tests for request flag
Fix test name
Merge pull request #62 from moeffju/feature/fix-travis-temp

Fix Travis build / fix tests (temporary fix)
Merge pull request #61 from moeffju/feature/minify-only-requested

Only process response if the request reached htmlmin