move config.assets.precompile into config/application.rb

Since `rake assets:precompile` is no longer executed in production it makes no sense to have the option `config.assets.precompile` in `config/environments/production.rb`. I moved it into `config/application.rb` since it makes more sense to configure what to precompile for the whole application than for different environments.

I'm not sure if this is the only option that needs to move. I already talked with @guilleiguaran about that change, so if further things come up, let me know.