Merge pull request #616 from junaruga/wip/rdoc-dev-dep

Add rdoc as a development dependency.

Author: hsbt

Diff for: Gemfile

@@ -6,4 +6,6 @@ group :development do
   gem "rake"
   gem "rake-compiler"
   gem "test-unit", "~> 3.0", ">= 3.4.6"
+  # In the case of Ruby whose rdoc is not a default gem.
+  gem "rdoc"
 end