Fixed issue #19 'Rake not auto completing'. The stat command syntax that was...

Fixed issue #19 'Rake not auto completing'.  The stat command syntax that was being used was not correct. Or at least on my machine.  See man page for the stat command

lib/rake_completion.zsh

 _rake_does_task_list_need_generating () {
   if [ ! -f .rake_tasks~ ]; then return 0;
   else
-    accurate=$(stat -f%m .rake_tasks~)
-    changed=$(stat -f%m Rakefile)
+    accurate=$(stat -c "%n" .rake_tasks~)
+    changed=$(stat -c "%n" Rakefile)
     return $(expr $accurate '>=' $changed)
   fi
 }