Release v1.2.3 - 2016-04-01

References: #20, #21

• Will no longer require json_pure if the json gem has already been loaded.
  This will result in a warning and a degraded experience if json < 1.8.1
  has already been loaded.

  Mixing json/pure with json/ext results in json errors, for example:

  some_hash = { 'jsonrpc' => 'abc', 'jsonversion' => 1 }
  some_hash.to_json
  #=> raises a JSON::Pure::Generator::State TypeError

  See related GitHub issue #20.