Remove json dependency

JSON is included in Ruby >= 1.9, and the gem is removed for 2.1.0, which means that things which implicitly depend on sxp fail on Ruby 2.1.0. Note that RBX does unbundle this, so a rubysl-json dependency is required for this platform.

Pulling this and pushing an updated gem would be useful for all those things which don't explicitly use my branch in a Gemfile.