Add heroku gems for rails 4, remove the initialize_on_precompile = false

Heroku no longer uses the initialize_on_precompile for rails 4.
(https://devcenter.heroku.com/articles/rails-asset-pipeline)

Heroku requires rails_12factor gem and if assets are not behind nginx
Heroku also requires rails_serve_static_assets gem as well
(https://devcenter.heroku.com/articles/ruby-support#injected-plugins)

This fixes all the asset precompile problems on heroku for me.