Update to make the parser case insensitive to HTML tags

According to the bug https://bugzilla.mozilla.org/show_bug.cgi?id=964331  CodeMirror parser was case -sensitive to HTML tags. A bug was filed towards CodeMirror repository https://github.com/marijnh/CodeMirror/issues/2304 and later it was fixed by the author. I have tested the update on a local Thimble copy and it works. Parser now is case-insensitive. I've updated the 2 files in Thimble.webmaker.org repository to implement the fix.