Downcase adapter string name for OS compatibility.

We had problems deploying to Ubuntu because I had used `MultiJson.use 'Oj'`. It worked on my OSX machine, but not on the Ubuntu machine because of how different OSes handle case-sensitivity. (I think my coworker had problems on linux as well) We changed our code to use a downcased version of the adapter, but this might help ease some pain in the future.