Update charts dependency (#11)

bower.json

@@ -19,7 +19,7 @@
   ],
   "dependencies": {
     "vaadin-board": "2.0.0-alpha4",
-    "vaadin-charts": "6.0.0-alpha14",
+    "vaadin-charts": "6.0.0-alpha15",
     "vaadin-core": "10.0.0-alpha18"
   }
 }