activeadmin: Bundler could not find compatible versions (Rails 4.1)

Attempting to install active admin using rails 4.1 I get this:

Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    activeadmin (>= 0) ruby depends on
      sprockets (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets (2.12.0)

Bundler could not find compatible versions for gem "sprockets-rails":
  In Gemfile:
    activeadmin (>= 0) ruby depends on
      sprockets-rails (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets-rails (2.1.1)

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    activeadmin (>= 0) ruby depends on
      railties (~> 3.1.0) ruby

    rabl-rails (>= 0) ruby depends on
      railties (4.1.0)

My Gemfile has this:

gem 'activeadmin', github: 'gregbell/active_admin'