Removes need to use babel-register (#4865)

* Removes need to use babel-register

- Adds watch to watch changes when running the test to regenerate
- Tests are now pure node 8

* Adds timing to helper.js

* Update contribution guide

* Adds inline sourcemaps generation to restore coverage

* nits

spec/PromiseRouter.spec.js

@@ -1,4 +1,4 @@
-const PromiseRouter = require("../src/PromiseRouter").default;
+const PromiseRouter = require("../lib/PromiseRouter").default;
 
 describe("PromiseRouter", () => {
   it("should properly handle rejects", (done) => {