Removed the Ruby encoder and switched to using the JSON gem

Got all the tests passing again.

There are two limitations of that's worth mentioning:

1. Support for `encode_json` has been removed (and consequently the
   ability to encode `BigDecimal`s as numbers, as mentioned in the
   previous commit). It will be moved to the ${GEM_NAME}.

2. Custom implementations of `as_json` on `Hash`, `Array`, `String`,
   `Numeric`, `TrueClass`, `FalseClass` and `NilClass` might not be
   picked up by the encoder. It is recommended that you subclass
   these base classes instead.