Setup greenkeeper to detect & update outdated dependencies

[bajtos]

GreenKeeper is a service that monitors outdated dependencies and can send a pull request to update them. It's a great tool for ensuring that we are always using the latest version of our dependencies.

In the past, it did not support monorepos and thus we could not use in for loopback-next. Fortunately, things have changed and GreenKeeper does support monorepos today.

Docs: https://greenkeeper.io/docs.html#monorepo

I am proposing to enable GreenKeeper for loopback-next.

See also the discussion in #2207

[dhmlau]

@bajtos , since the requirement is originated from the example app, I'm thinking to set up GreenKeeper in this task as well.

Acceptance Criteria

Follow instruction in https://greenkeeper.io/docs.html#monorepo, set up GreenKeeper for:

• loopback-next
• loopback4-example-shopping

[raymondfeng]

See #2325

[raymondfeng]

And #2323

[dhmlau]

@raymondfeng , since we have greenkeeper now, would this task be considered as resolved?

[raymondfeng]

Done.