Update app_builder.rb

In Rails 4, 'public/index.html' and 'rails.png' files are don't exist. A new Rails internal controller Rails::WelcomeController has been used to serve the dynamic welcome screen and rails.png image used as data-image. So I removed these lines from the file.
Remove unnecessary builds

Removed 'remove_public_index' and 'remove_rails_logo_image' methods.
Merge pull request #43 from sbagdat/patch-2

Update app_builder.rb thx @sbagdat