Fix requirements

Dependencies are now specified in setup.py. This is good practice and allows using dynamic specs depending on Django and Python versions. Thus, requirements.txt can be used for installation specific requirements. Also, .travis/requirements.txt became unnecessary and was removed. In addition, .travis.yml became simpler because version-dependent requirements are already handled in setup.py, thus it is necessary to only specify Django and Python versions.

Contributions:
 * fix issue https://github.com/django-wiki/django-wiki/issues/319
 * clean a bit messy dependency definitions in requirements.txt and setup.py (and remove .travis/requirements.txt)
 * fix unit tests for Python 2 (Travis shows green light for Python 2 and Django<1.7)