Fix Capistrano warnings: Missing public directories

I got these warnings while deploying a Rails 3.2 app with asset pipeline enabled:

*** [err :: host.tld] find: `/opt/www/hollyapp.com/releases/20120503115342/public/images': No such file or directory
*** [err :: host.tld] find: `/opt/www/hollyapp.com/releases/20120503115342/public/stylesheets': No such file or directory
*** [err :: host.tld] find: `/opt/www/hollyapp.com/releases/20120503115342/public/javascripts': No such file or directory

Folders like public/javascripts might not exist if you're using the asset pipeline (everything lives in app/assets).

I got rid of the warnings by committing empty files into Git:

• public/images/.gitkeep
• public/javascripts/.gitkeep
• public/stylesheets/.gitkeep