I tried this tip: https://github.com/thoughtbot/laptop/pull/14#issuecomment-3192270 .
On deploy I see
-----> Using RUBY_VERSION: ruby-1.9.3-p0
Here's an update for everyone referencing this question... Heroku now allows you to specify your ruby version in your Gemfile, thanks to their addition to the latest version of bundler.
First run:
gem install bundler --pre
then add ruby '1.9.3' to your Gemfile as such.
source 'http://rubygems.org'
ruby '1.9.3'
gem 'rails', '3.2.3'
The full release from Heroku with details is at http://blog.heroku.com/archives/2012/5/9/multiple_ruby_version_support_on_heroku/