Remove warnings from code

[humitos]

This PR removes almost all the WARNING messages when running tests:

• migrate render_to_response to render from Django
• use prepend_urls instead override_urls from tastypie

There are two more warnings:

1. Tastypie uses an old method from Django, but we can't upgrade (even from 0.13.0 to 0.13.3) because there are some tests that do not pass (I could take a look at this in another PR)

readthedocs/rtd_tests/tests/test_api.py::APITests::test_make_project
  /home/humitos/rtfd/code/readthedocs.org/.tox/py27/lib/python2.7/site-packages/tastypie/resources.py:2116: RemovedInDjango110Warning: 'get_all_field_names is an unofficial API that has been deprecated. You may be able to replace it with 'get_fields()'
    field_names = self._meta.object_class._meta.get_all_field_names()

2. Linaro Django Pagination uses context_instance for render_to_string. I took a look at the code but we are using an specific commit directly from git and I found that project was forked and abandoned a couple of times. So, I'm not sure if it's safe to update and to what version

readthedocs/rtd_tests/tests/test_privacy.py::PrivacyTests::test_build_filtering
  /home/humitos/rtfd/code/readthedocs.org/.tox/py27/lib/python2.7/site-packages/linaro_django_pagination/templatetags/pagination_tags.py:199: RemovedInDjango110Warning: The context_instance argument of render_to_string is deprecated.
    context_instance = context)

[ericholscher]

1. We should deprecate our tastypie API and remove it.
2. Not sure about.

This is a solid 👍 from me once the tests pass :)

[humitos]

Test are passing, the problem is the linting. I wasn't sure if I should apply all the tools we have to this files or not.

Since, you already review it, I could apply them and make a new commit with all of the files involved.

1. and 2) could be in another PR I think.

[ericholscher]

LGTM. Will likely cause a bunch of other PR's to conflict though :(