This would be useful:
ruby '~> 1.9.3'
+1 on this
This isn't possible to do in a backwards compatible way since we cannot alter the lock file parser to work correctly. Ref: #2228
We can revisit this once we decide to move to ruby 2. Please re-open on https://github.com/bundler/bundler-features if you want this to be tracked somewhere.
s/ruby 2/bundler 2/?