Add support for RubyGems >= 2.0.0

RubyGems 2.0.0preview2 removed --no-rdoc and --no-ri options
for the install command.

(https://github.com/rubygems/rubygems/blob/master/History.txt)

rubygems.py

@@ -203,8 +203,6 @@ class Recipe(object):
             self.ruby_executable,
             gem_executable,
             'install',
-            '--no-rdoc',
-            '--no-ri',
             '--bindir=%s' % bindir,
         ]