You will need to use Ruby 1.9.3. See Mongoid documentation here and Heroku documentation here on how to use 1.9.3.
I was also getting similar errors, and I changed my gemfile to include ruby '1.9.3' and everything worked as expected again.
Edit:
@herb pointed out that you need the latest version of bundler for this to work (gem install bundler --pre). Otherwise, ruby '1.9.3' in your Gemfile will not work.