Turn off JS Minification for Heroku

Given the outcome of [#123] and [#124], we don't take a stand on where
minification should happen.

However, to make sure that Heroku deployments don't take too long, the
generator can disable Rails-side minification.

The benefit of doing this in the generator (as opposed to automatically
in [the addon]) is that the change will introduce diff noise, which is
explicit and easy to back out of.

[#123]: #123
[#124]: #124
[the addon]: https://github.com/rondale-sc/ember-cli-rails-addon/blob/2f2a47efe6e81a2bcd43ec9ff0f89fc18bed5a03/index.js#L25-L32

lib/generators/ember-cli/heroku/heroku_generator.rb

@@ -13,6 +13,16 @@ def copy_setup_heroku_file
       run "chmod a+x bin/heroku_install"
     end
 
+    def config_js_compressor
+      production_config = "config/environments/production.rb"
+
+      inject_into_file production_config, before: "end\n" do
+        <<-RUBY
+  config.assets.js_compressor = nil
+        RUBY
+      end
+    end
+
     def inject_12factor_gem
       gem "rails_12factor", group: [:staging, :production]
     end