Revert "Upgrade jest for it to work with babel@7"

This reverts commit 130f8f6.

Author: vhfmag

package.json

@@ -25,7 +25,7 @@
     "eslint": "^3.12.1",
     "eslint-config-airbnb-base": "^11.1.0",
     "eslint-plugin-import": "^2.2.0",
-    "jest": "^21",
+    "jest": "^20.0.0",
     "rimraf": "^2.5.2"
   },
   "engines": {