Report test summaries to CircleCI (#4704)

* Report test summaries to CircleCI

* Don’t run jest-junit on Node <6

(it uses default function parameters)

* Address @lydell’s comment

* Use @duailibe’s suggestion

* 🔥 `jest-junit` key in production package.json

* Move config to jest.config.js

.gitignore

@@ -17,3 +17,4 @@
 coverage
 .idea
 package-lock.json
+/test-results