hard-coded git_url to default attribute

"oh-my-zsh" git repository URL is hard-coded on recipes/default.rb and URL scheme is "git://".
```
git "#{home_directory}/.oh-my-zsh" do
    repository 'git://github.com/robbyrussell/oh-my-zsh.git'
```
Change this URL to "default attribute", then users behind firewall and proxy can override and "git clone" works.