Update responsiveness.js

Changing allowed delay from 300 to 400 because Safari OSX is consistently slow compared to every other modern browser.

tests/frontend/specs/responsiveness.js

@@ -77,7 +77,7 @@ describe('Responsiveness of Editor', function() {
         var end = Date.now(); // get the current time
         var delay = end - start; // get the delay as the current time minus the start time
 
-        expect(delay).to.be.below(300);
+        expect(delay).to.be.below(400);
         done();
       }, 1000);