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
According to the Heroku Cedar stack definition here Cedar currently only supports MRI 1.9.2. Have you tried contacting Heroku support? They're generally pretty responsive.
Edit: it's possible to 'fool' the platform sometimes into running some binaries such as running Haskell on Heroku so if you're feeling really dedicated you might be able to package something up. I imagine this isn't too easy with MRI though.