Use tox generative envlists

http://testrun.org/tox/latest/config.html#generating-environments-conditional-settings
Test all supported versions of Django
Update Travis list of tox environments

(`tox -l` + copypaste...)
Drop Python 3 tests for Django 1.4

(Given that Django 1.4 didn't support Python 3, these were never going
to go well)
Merge branch 'develop' into feature/multidjango
Test Django dev version (but ignore failure)
Drop Python 3 Django 1.4 tests from Travis matrix

(should have been included in b8eeadad43f47ff45e1d4cdbc6a2baed3f02bbf8)
Added CHANGELOG notes about Django version support
Merge pull request #10 from laterpay/feature/multidjango

Test all supported versions of Django