RailsApps/rails-composer — pull request #78 by dalton (2013-02-20 15:13:56+00:00)
Title: binding_of_caller only works in MRI 1.9.2-3 and RBX

I added a :platforms group to the binding_of_caller gem to prevent it from being installed in non-MRI/Rubinius Ruby implementations (JRuby, et al)

https://github.com/banister/binding_of_caller

http://gembundler.com/man/gemfile.5.html