[TechDebt] Remove GreenKeeper

[ctsstc]

Looks like it's going away: https://greenkeeper.io/

We can move over to Snyk like they recommend or just roll with the newer github security junk that's built in now (I don't remember what that's called off hand right now).

I say we give Snyk a try though : )

Edit: I've requested access to add it.

RIP

[ctsstc]

Closed with #128

I do not think any additional work is required. There are some pending PRs it opened, that should be checked out, but those may fall in line with performing some major dependency updates noted in #118