Create this file, and host it on your servers somewhere:
# our_defaults.rb
run "grep -v '/bin' .gitignore > .gitignore.new && mv .gitignore.new .gitignore"
On the command line, when you create a new Rails app, do this:
rails new my_application_name -m https://dev.mycompany.com/our_defaults.rb
The long version: http://gilesbowkett.blogspot.com/2013/01/we-can-solve-multiple-default-stacks.html