fix weird scroll bug

Overriding .Codemirror-scroll to overflow: auto fixes the weird choppy scrolling mentioned in this requirebin issue: https://github.com/maxogden/requirebin/issues/10

Tested on Mac with Chrome and Firefox.

Found the fix by updating/digging through the latest codemirror.css, but updating the whole file also introduces other weird formatting issues, so I figure this little override is probably good for now.