to_json behaves strangely in rails 4.0 -- Use `JSON.generate 'html', quirks_mode: true` to get consistent behavior

Rails decided to make `to_json` escape </> with 6 character unicode escape sequences. Ew. We're trying to compress, not expand our assets.
Rails production minified swapped out quotes.  Since the integration tests better cover this anyways, trash the assertion. We don't want to  test the implementation of the minification
Rails <4.0 use minitest 4, and rails 4.1 uses minitest 5. be loose in the gemspec, and add forwards compat to the test_helper