Update vaadin-charts version to alpha7 (#5)

bower.json

@@ -19,7 +19,7 @@
   ],
   "dependencies": {
     "vaadin-board": "1.1.0-beta1",
-    "vaadin-charts": "6.0.0-alpha4",
+    "vaadin-charts": "6.0.0-alpha7",
     "vaadin-core": "10.0.0-alpha9"
   }
 }