chore(package): update jest to version 21.0.2

Closes #25

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
   "devDependencies": {
     "@types/jest": "^20.0.5",
     "del-cli": "^1.1.0",
-    "jest": "^20.0.4",
+    "jest": "^21.0.2",
     "ts-jest": "^20.0.7",
     "tslint": "^5.5.0",
     "tslint-config-airbnb": "^5.2.1",