Fix issue #1598 by adding a dependency to the RDoc gem.

railties/railties.gemspec

@@ -20,6 +20,7 @@
   s.add_dependency('rake',          '>= 0.8.7')
   s.add_dependency('thor',          '~> 0.14.6')
   s.add_dependency('rack-ssl',      '~> 1.3.2')
+  s.add_dependency('rdoc',          '~> 3.4')
   s.add_dependency('activesupport', version)
   s.add_dependency('actionpack',    version)
 end