fix(intersectionObs): guarding against undefined window (#26)

Author: ashbrowning

src/utils/intersection-observer.js

@@ -1,5 +1,6 @@
 export default function() {
   return (
+    typeof window !== 'undefined' &&
     'IntersectionObserver' in window &&
     'isIntersecting' in window.IntersectionObserverEntry.prototype
   );