Removed the Ruby encoder and switched to using the JSON gem

Got all the tests passing again.

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}.
Make the JSON encoder pluggable
Process::Status should get a :nodoc: [ci skip]
Be explicit and use the actual unicode sequence