Listing dependencies in the default group & Multi_JSON

* Using [Multi_JSON](https://rubygems.org/gems/multi_json) so we always load the fastest possible JSON coder.
* Fixing the gemspec to list:
    * sequel
    * activerecord
    * pg
    * connection_pool

These should belong in the default group not development as they are stedfast requirements for the gem to function.

Ran all the tests and everything passed :smile: