There seems to be an issue with Ruby 1.9.2 on Heroku and how may bytes it thinks int is.
Try to add ruby "1.9.3"
to you Gemfile like this:
source :rubygems
ruby "1.9.3"
gem 'sinatra'
gem 'thin'
gem 'haml'
gem 'rdiscount'
My own testing reproduced your error on 1.9.2 and succeeded on 1.9.3