heroku run rails c
Running `rails c` attached to terminal... up, run.1879
/app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/dependency.rb:296:in `to_specs': Could not find 'sprockets' (~> 2.2.1) among 131 total gem(s) (Gem::LoadError)
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1196:in `block in activate_dependencies'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1185:in `each'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1185:in `activate_dependencies'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1167:in `activate'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1199:in `block in activate_dependencies'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1185:in `each'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1185:in `activate_dependencies'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb:1167:in `activate'
from /app/vendor/ruby-2.0.0/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_gem.rb:48:in `gem'
from /app/vendor/bundle/ruby/2.0.0/bin/rails:22:in `<main>'
Note: I am based on master as of today - I suspect (but have not confirmed) that it's related to the newish plugin booting process.