Update dependency lerna to v3

[eliperelman]

See #1018.

[edmorley]

It appears that none of the breaking changes here affect us. I'll try and dig a little deeper.

The error comes from:
https://github.com/lerna/lerna/blame/f2c470a7d33e20be3087c0b0e5e052a601863b82/commands/version/index.js#L87

...which was added in:
lerna/lerna#1522

Note that Travis uses a weird way of checking out commits, which means the check fails:
https://travis-ci.org/mozilla-neutrino/neutrino-dev/jobs/416449876#L445

[edmorley]

I'm going to try and have another go with the most recent 3.x lerna release.

I think a combination of lerna version and lerna publish from-git might allow us to have a multi-stage workflow where we first open a PR with the version bumps, that means we can review the changes etc, before then merging it and using publish to actually publish the changes.

https://github.com/lerna/lerna/tree/master/commands/version#lernaversion
https://github.com/lerna/lerna/tree/master/commands/publish#lernapublish

[eliperelman]

Closing in favor of #1311.