Fix rendering of text before font load (close #16)

static/canvas.js

@@ -181,9 +181,9 @@ window.addEventListener('load', function() {
   }
 
   // Update canvas
-  document.fonts.ready.then(function() {
+  document.fonts.onloadingdone = function(fontFaceSetEvent) {
     draw();
-  });
+  }
 });
 
 let randomElement = function(selector) {