Merge pull request #45 from reactive-react/greenkeeper/@types/jest-21.1.0

Update @types/jest to the latest version 🚀

Author: jcouyang

package.json

@@ -36,7 +36,7 @@
   },
   "devDependencies": {
     "@reactivex/rxjs": "^5.4.3",
-    "@types/jest": "^20.0.2",
+    "@types/jest": "^21.1.0",
     "@types/node": "^8.0.0",
     "@types/prop-types": "^15.5.2",
     "@types/react": "^16.0.13",