[api] fix performance problem when rendering json

as it turned out rails is forcing a specific (slow) json render, so
it doesn't matter how fast the json gem is if it's not used. So
overwriting the render used for render: json to use fast_generate

Before project_infos/openSUSE:Factory: 1115ms - after 765ms, so it saves
on that project (with quite some packages) 350ms, which is not bad for
3 lines

Unfortunately JSON.generate can't be used within rails app due to
https://github.com/intridea/multi_json/issues/86 nonsense, so switch
back to yajl