Update json gem version to 1.8.1

Update json gem version to 1.8.1

Changes of Json 

* Fix regex encoding problem on ruby-head, ouch
* Remove Rubinius exception since transcoding should be working now.
* Try to convert first with to_hash, then to_h rb_convert_type doesn't return if conversion fails, so use rb_check_convert_type and the raise vi rb_convert_type. Make sure, that this behaviour is consisten across all generator implementations.
* also ruby 2.0 and some more