Fix deletion of multiple lines at once in CodeMirror

This fixes the deletion of **multiple** lines in CM. I think it also is a fix for https://github.com/josephg/ShareJS/issues/182 and definitely fixes the bug mentioned in https://github.com/josephg/ShareJS/pull/204#issuecomment-17756782.

Again, I'm sorry for not testing the original PR properly.. the version upgrade seems to have increased the effect of the original bug.. but now it's fixed. I hope.