Explicitly require rails components but not active_resource

This app doesn't use active_resource so we might as well not load it. This reduces
the surface area of the app, may slightly reduce memory consumption, and is pleasantly
explicit.
Disable JSON and XML parameter parsing

We're already doing this in production using configuration added at deploy time.
It's better to have that happen in the app to keep development and production
more in sync.
Uncomment param parser removal

Thanks @jamiecobbett - I had commented them for testing and forgot to reinstate
Merge pull request #55 from alphagov/remove-unneeded-components

Remove unneeded components