Switch back to YAJL for JSON-encoding.

A part of switching away from RABL is that we lost its configuration file,
which configured it to use YAJL to encode its JSON[1]. Reinstating YAJL should
make JSON encoding faster and less hungry for memory.

[1]: https://github.com/alphagov/govuk_content_api/blob/248a26df/config/rabl.rb