Plugin: Rake-fast: Support both *nix and Darwin

plugins/rake-fast/rake-fast.plugin.zsh

@@ -27,8 +27,13 @@ _rake_refresh () {
 _rake_does_task_list_need_generating () {
   if [ ! -f .rake_tasks ]; then return 0;
   else
+    if [[ $(uname -s) == 'Darwin' ]]; then
       accurate=$(stat -f%m .rake_tasks)
       changed=$(stat -f%m Rakefile)
+    else
+      accurate=$(stat -c%Y .rake_tasks)
+      changed=$(stat -c%Y Rakefile)
+    fi
     return $(expr $accurate '>=' $changed)
   fi
 }