fetch districts with ajax GET for speediness

‘POST requests are never cached’: http://blog.httpwatch.com/2009/08/07/ajax-caching-two-important-facts/

For users that have visited previously, we can speed loading by a factor of ten by switching from doing the AJAX request via POST to GET (which allows caching). You can see this by looking at Chrome’s dev console -> ‘network’ tab and observing a page load.
Merge pull request #124 from codeforamerica/bugs/improve-response-speed

fetch districts with ajax GET for speediness