Remove trailing commas for IE 9 compatibility

Summary: IE 9 doesn't like trailing commas. When we introduced eslint, we added
a bunch of trailing commas, which makes IE 9 sad.

Test Plan:
 - `make lint`
 - Visit http://localhost:7936/ using IE 9 on browserstack.
 - See that the math loads, and there are no errors in the F12 developer tools.

@kevinb

katex.js

@@ -70,5 +70,5 @@ module.exports = {
      * to change. Use at your own risk.
      */
     __parse: generateParseTree,
-    ParseError: ParseError,
+    ParseError: ParseError
 };