Pin jest 21 until jestjs/jest#5109 is resolved

Author: nevir

package.json

@@ -51,7 +51,7 @@
     "eslint-plugin-typescript": "^0.8.1",
     "graphql": "^0.11.7",
     "graphql-tag": "^2.4.2",
-    "jest": "^22.0.0",
+    "jest": "^21.2.1",
     "jest-junit": "^3.4.0",
     "rimraf": "^2.6.1",
     "typescript": "~2.6.2",