Log Message:
Update ruby-json to 1.5.1.

2011-01-24 (1.5.1)
  * Made rake-compiler build a fat binary gem. This should fix issue
    https://github.com/flori/json/issues#issue/54.
2011-01-22 (1.5.0)
  * Included Java source codes for the Jruby extension made by Daniel Luz
    <dev@mernen.com>.
  * Output full exception message of deep_const_get to aid debugging.
  * Fixed an issue with ruby 1.9 Module#const_defined? method, that was
    reported by Riley Goodside.