Update with-mobx example (#5229)

Update to new @babel/plugin-proposal-decorators, @babel/plugin-proposal-class-properties plugins version.

examples/with-mobx/.babelrc

@@ -3,7 +3,7 @@
     "next/babel"
   ],
   "plugins": [
-    "transform-decorators-legacy",
-    "transform-class-properties"
+    ["@babel/plugin-proposal-decorators", { "legacy": true }],
+    ["@babel/plugin-proposal-class-properties", { "loose": true }]
   ]
 }

examples/with-mobx/package.json

@@ -15,7 +15,7 @@
   },
   "license": "ISC",
   "devDependencies": {
-    "babel-plugin-transform-class-properties": "^6.24.1",
-    "babel-plugin-transform-decorators-legacy": "^1.3.4"
+    "@babel/plugin-proposal-class-properties": "^7.1.0",
+    "@babel/plugin-proposal-decorators": "^7.1.0",
   }
 }