readme: Use anonymous git url for Ubuntu and Mac
readme: Use anonymous git url for Ubuntu and Mac

The Windows section uses this already.

The authenticated url will fail if the user doesn't have access
set up for the Git.
biodamasceno/GitGutter,akpersad/GitGutter,natecavanaugh/GitGutter,robfrawley/sublime-git-gutter,akpersad/GitGutter,michaelhogg/GitGutter,natecavanaugh/GitGutter,biodamasceno/GitGutter,akpersad/GitGutter,robfrawley/sublime-git-gutter,natecavanaugh/GitGutter,michaelhogg/GitGutter,robfrawley/sublime-git-gutter,biodamasceno/GitGutter,jisaacks/GitGutter,michaelhogg/GitGutter,natecavanaugh/GitGutter,michaelhogg/GitGutter,robfrawley/sublime-git-gutter,tushortz/GitGutter,tushortz/GitGutter,akpersad/GitGutter,tushortz/GitGutter,tushortz/GitGutter,biodamasceno/GitGutter