fix(package): update mobx to version 4.0.1

Closes #14

Author: greenkeeper[bot]

package.json

@@ -12,7 +12,7 @@
   "license": "MIT",
   "dependencies": {
     "express": "^4.13.4",
-    "mobx": "^3.2.1",
+    "mobx": "^4.0.1",
     "mobx-react": "^4.2.2",
     "react": "^15.6.1",
     "react-dom": "^15.6.1",