Log Message:
Update ruby-json and ruby-json-pure to 1.7.3.

2012-05-12 (1.7.3)
  * Work around Rubinius encoding issues using iconv for conversion instead.
2012-05-11 (1.7.2)
  * Fix some encoding issues, that cause problems for  the pure and the
    extension variant in jruby 1.9 mode.
2012-04-28 (1.7.1)
  * Some small fixes for building