Add a basic PUT endpoint that returns a 200 status code

- This is to check that we've set it up correctly. This isn't going to
  be the final product.
Strip out Rails' default .json/.xml format file extensions

- We want clean URLs, with Content-Type specified by headers, not .json
  or .xml.
Merge pull request #3 from alphagov/add_basic_put_endpoint

Add a basic PUT endpoint