Remove json gem
Remove json gem

JSON support is in Ruby since 1.9 so there is no need to have json gem
as a dependency.

Inspired by this Rack’s PR https://github.com/rack/rack/pull/1011

jnunemaker/httparty,ffleming/httparty,ffleming/httparty,jnunemaker/httparty,jnunemaker/httparty,ffleming/httparty