How to get a specific "commit" of a gem from github?

I'm using rails_admin, and since it is in (very) active development, bugs turn up every now and then.

There are no versions for the gem as far as I can tell, for the gem in github, so I can't use the :version key for the gem declaration in the Gemfile .

Is there a way I can "tie" a specific commit(that I know is working fine for me) to the Gemfile ?

I currently have in my Gemfile:

gem 'rails_admin', 
  :git => 'git://github.com/sferik/rails_admin.git'

I'd like to be able to do something like this (example "commit_id"):

gem 'rails_admin', 
  :git => 'git://github.com/sferik/rails_admin.git',
  :commit_id => "4e7d53e3c5c4c3c5c43c3"

Is this possible to do with github?