Heroku deployment with active_admin
Looks like the rails4 branch for active_admin was removed!
You can update your gemfile and specify a tag or commit reference. As of today v0.6.1 is the most up to date version, so you could do:
gem "activeadmin", github: "gregbell/active_admin", tag: "v0.6.1"..and that would solve your problem. Alternatively, as you pointed out in your comment, choosing another specific revision would also get rid of the error.
gem "activeadmin", github: "gregbell/active_admin", ref: "82ff48d51525bdf55c96e50d8650d45f2a6716ed"