Update dependency ts-jest to v21.2.4

Author: renovate-bot

package.json

@@ -17,7 +17,7 @@
     "@types/parse": "2.4.4",
     "jest": "22.4.2",
     "parse": "1.11.0",
-    "ts-jest": "21.1.3",
+    "ts-jest": "21.2.4",
     "tslint": "5.8.0",
     "tslint-config-airbnb": "5.3.0",
     "typescript": "2.5.3"