fixing shipshapecode#217 - performance.now is null

causing 

Uncaught TypeError: Object [object Performance] has no method 'now',

It has been fixed the same in other projects as well

src/js/tether.js

@@ -47,7 +47,7 @@ const position = () => {
 };
 
 function now() {
-  if (typeof performance !== 'undefined' && typeof performance.now !== 'undefined') {
+  if (performance && performance.now) {
     return performance.now();
   }
   return +new Date;