upgrade extend, fixes #2638

Author: jhchen

package.json

@@ -35,7 +35,7 @@
     "clone": "^2.1.1",
     "deep-equal": "^1.0.1",
     "eventemitter3": "^2.0.3",
-    "extend": "^3.0.1",
+    "extend": "^3.0.2",
     "parchment": "^1.1.4",
     "quill-delta": "^3.6.2"
   },