Don't initialize when compiling assets to avoid exception on Heroku

Heroku runs rake assets:precompile when deploying a Rails app.

Heroku needs initialize_on_precompile to be false otherwise it throws an exception during deployment (see http://guides.rubyonrails.org/asset_pipeline.html#precompiling-assets), if we add this option, we can remove one task in the https://github.com/fdv/typo/wiki/Installing-on-Heroku page :+1: .