Log Message:
Update ruby-json to 1.8.0.

2013-05-13 (1.8.0)
  * Fix https://github.com/flori/json/issues/162 reported by Marc-Andre
    Lafortune <github_rocks@marc-andre.ca>. Thanks!
  * Applied patches by Yui NARUSE <naruse@airemix.jp> to suppress warning with
    -Wchar-subscripts and better validate UTF-8 strings.
  * Applied patch by ginriki@github to remove unnecessary if.
  * Add load/dump interface to JSON::GenericObject to make
        serialize :some_attribute, JSON::GenericObject
    work in Rails active models for convenient SomeModel#some_attribute.foo.bar
    access to serialised JSON data.