Add rdoc as a development dependency.

There is a case of the Ruby where the rdoc is not a default gem. That is Ruby
RPM packages on Fedora Linux and the downstream Linux. The intention is to
reduce the size of the package.

Author: junaruga

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