Heroku's article about specifying ruby versions say this:
If you’re using a Bundler 1.1.4 or lower you’ll see the following error:
undefined method `ruby' for # (NoMethodError)
You’ll need to install bundler 1.2.0 or greater to use the ruby keyword.
I think you just need to update your local bundler to 1.2.0 or greater, as mine (1.3.5) recognize the ruby directive just fine as well.