FIXES #78649336 - heroku wont be able to make use of arrays fed to Setti...

PT: https://www.pivotaltracker.com/story/show/78649336

as https://github.com/railsconfig/rails_config states concerning arrays  in section heroku: 
"It won't work with arrays, though."
So for now just split the string..
does it need to be `ENV` here? why not a `let`?