add support to mobx v6 in mst devtools (#156)

Author: moharamfatema

packages/mobx-devtools-mst/package.json

@@ -23,7 +23,7 @@
         "build": "cross-env NODE_ENV=production webpack"
     },
     "peerDependencies": {
-        "mobx": "^2.2.0 || ^3.0.0 || ^4.0.0 || ^5.0.0",
+        "mobx": "^2.2.0 || ^3.0.0 || ^4.0.0 || ^5.0.0 || ^6.0.0",
         "mobx-state-tree": "*"
     }
 }