Serve source map files as `application/json`

Developer tools don't actually care what `Content-Type` source map
files are served with. However, mapping their file extension (`.map`)
to the `application/json` media type, ensures that they will be served
compressed and their character encoding will be UTF-8.

See also:

* Source Map Revision 3 Proposal
  https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/

* Introduction to JavaScript Source Maps
  http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/

* How CSS source maps work
  https://developers.google.com/chrome-developer-tools/docs/css-preprocessors#toc-how-css-source-maps-work

* What MIME-type should I use for source map files?
  http://stackoverflow.com/a/19912684