Fix templates non rendering, because of missing template suffix

This seems to regressed on the latest ActionView, which doesn't seem to
render plain `html` files with ERB expansions.

I initially dropped the `.erb` part with the hope that more designers
can contribute to the project, having it nicely highlighted in an
arbitrary editor.

The reason, I didn't catch this early was because the master Gemfile
wasn't bundling on the CI and I'm trying to fix that as well, so I can
prevent other regressions in future.

Author: gsamokovarov

Gemfile

@@ -3,6 +3,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'rails', github: 'rails/rails'
+gem 'arel', github: 'rails/arel'
 
 platforms :jruby do
   gem 'activerecord-jdbcsqlite3-adapter'