Use Oj correctly as the JSON engine

Though Oj was originally intended to be used (hence it was in the Gemfile), it was not being used by all of Rabl, AMS, and presenters.

In fact, it was only being used by Rabl as it is the engine Rabl uses by default. In othewords, the benchmark was unfair to AMS and presenters. With the Oj boost to AMS and presenters, the relative performance of Rabl gets even worse.

**NOTE**: I will update the blog post (http://techblog.thescore.com/benchmarking-json-generation-in-ruby) with up-to-date benchmark results once this gets merged into master.