[V1] Upgrading server framework

[kriswep]

Gatekeeper uses express v3 as server framework.
This needs to be updated to v4.

Alternatively, we could consider using an alternative framework.
I would suggest (micro)[https://github.com/zeit/micro]. It's a lightweight framework, which focuses on new ES2017 syntax and works nicely with async/await. Would work nicely for the few route endpoints we need.