Add json gem as a development dependency

RubyGems' rubygems/test_case tries to activate 'json' gem explicitly.

Author: rhenium

rdoc.gemspec

@@ -52,4 +52,5 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   s.add_development_dependency("racc", "> 1.4.10")
   s.add_development_dependency("kpeg")
   s.add_development_dependency("minitest", "~> 4")
+  s.add_development_dependency("json")
 end