fix: update peer/dev dependency versions of mobx to v5 (#56)

* Updated package peer dependencies to support compatibility with MobX 5

* Updating dev dependency to latest mobx version

package.json

@@ -66,15 +66,15 @@
     "eslint-plugin-react": "^6.3.0",
     "eslint-plugin-standard": "^2.0.0",
     "jest": "^19.0.0",
-    "mobx": "^4.0.0",
+    "mobx": "^5.1.0",
     "react-router": "^4.0.0",
     "rimraf": "^2.5.4",
     "semantic-release": "^4.3.5",
     "typescript": "^2.2.2",
     "webpack": "^2.3.2"
   },
   "peerDependencies": {
-    "mobx": "^3.0.0 || ^4.0.0",
+    "mobx": "^3.0.0 || ^4.0.0 || ^5.0.0",
     "react-router": "4.x"
   }
 }