Add public/tinymce/skins/ to .gitignore

The `public/tinymce/skins/` directory and its contents are auto-generated whenever we run `rails s`.

Author: aaronskiba

.gitignore

@@ -14,6 +14,7 @@ public/assets/*
 public/fonts/*
 public/images/*
 public/javascripts/*
+public/tinymce/skins/
 public/stylesheets/*
 public/system/*
 public/videos/*