Only use plugin_dir in ST3
Only use plugin_dir in ST3

This will prevent ST2 from using GitGutter-EDGE
but at least git gutter will work in ST2 until
I figure out a way to get __file__ to work correctly
tushortz/GitGutter,robfrawley/sublime-git-gutter,natecavanaugh/GitGutter,jisaacks/GitGutter,robfrawley/sublime-git-gutter,natecavanaugh/GitGutter,natecavanaugh/GitGutter,natecavanaugh/GitGutter,michaelhogg/GitGutter,biodamasceno/GitGutter,biodamasceno/GitGutter,biodamasceno/GitGutter,akpersad/GitGutter,robfrawley/sublime-git-gutter,robfrawley/sublime-git-gutter,biodamasceno/GitGutter,tushortz/GitGutter,michaelhogg/GitGutter,michaelhogg/GitGutter,akpersad/GitGutter,tushortz/GitGutter,tushortz/GitGutter,akpersad/GitGutter,akpersad/GitGutter,michaelhogg/GitGutter