When you are getting

no implicit conversion of String into Integer
/Users/robin/.rvm/gems/ruby-2.0.0-p0/bundler/gems/requirejs-rails-f2330104aeca/lib/requirejs/rails/config.rb:140:in `[]'
/Users/robin/.rvm/gems/ruby-2.0.0-p0/bundler/gems/requirejs-rails-f2330104aeca/lib/requirejs/rails/config.rb:140:in `module_name_for'
/Users/robin/.rvm/gems/ruby-2.0.0-p0/bundler/gems/requirejs-rails-f2330104aeca/lib/requirejs/rails/config.rb:145:in `module_path_for'
(erubis:19:in `block in get_binding'
(erubis:18:in `each'
(erubis:18:in `get_binding'

it might be because you got YAML wrong. Here is what I had:

modules:
  -name: application
  -name: landing

And what it should have been:

modules:
  - name: application
  - name: landing