enable greenkeeper

[brucewpaul]

It would be great to enable greenkeeper so that we as major versions of packages that are in many peerdepencies get udpated, it's not as much of a pain to update to allow them (see #2091)

It's would also be great to keep dependencies updated in general, and this would help with that.

[Hypnosphi]

Greenkeeper doesn't support multiple packages in a repo, so we use dependencies.io for deps management. Initially, I wanted to enable issue creation for major versions bump, but @shilman and @danielduan had some objections

[danielduan]

Given the amount of dependencies we have, I think it'd be pretty noisy. I think greenkeeper opened an issue every few days the last time we had it turned on.

I am open to trying it out again though?

[Hypnosphi]

The check period is adjustable. It's 1 day currently, but we can change it to a week

[danielduan]

I'm fine with every week as long as someone is reviewing them.

[danielduan]

fixed with #2141