Cleanup & Suggestions

[rpkilby]

Hi all. I'd like to suggest a few changes, each detailed in individual commits below. Most of them are pretty minor, but I kept them separate for readability's sake.

The only changes that are likely controversial are:

• Removing requirements.txt in favor of inlining the contents of install_requires. Given that DRF is the only requirement, the file inclusion/parsing seems like overkill.

• Removing the DRF version bounds. From the commit message:

  New versions of DRF are more likely than not to be compatible. Instead
  of pessimistically restricting compatibility with future DRF versions,
  compatibility should be provided through documentation and explicit
  version bounds in test.

  It might be worth adding a note along the lines of "djangorestframework-gis is generally compatible with modern versions of Django REST Framework, however it's only explicitly tested against the latest version of DRF at the time of release, indicated in the compatibility matrix below."

  Edit: it looks like there was a compatibility issue when upgrading DRF 3.9. I don't fully understand the issue, but my understanding is that it was related to test output only, and doesn't reflect a change in compatibility or expected output from djangorestframework-gis.

[rpkilby]

Rebased off of #173, which fixes the test issue w/ DRF 3.9.x