When used in Production, JS wasn't loading

There seems to be a problem using this with a Rails app in `production`; everything is fine in `development` and `staging` but in production the `chartkick.js` file was served as an empty file but with a 200, resulting in no graphs :frowning:. This looks like a result of a misconfig of the Sinatra asset pipeline? This branch uses a simplified strategy which is working happily in Production.

(Graphs are awesome)