haml with markdown claims rdiscount not found

Question

I've specified

gem 'haml'
gem 'rdiscount'

in my Gemfile and ran bundle install

The relevant code is

.welcome-message
  :markdown
    Welcome **#{current_user.email}**

When I access the page I get

Can't run Markdown filter; required 'rdiscount' or 'peg_markdown' or 'maruku' or 'bluecloth', but none were found

I'm using rails 3.0.5, what am I missing?