[WIP] Update to lerna@2.1.1; use Yarn + workspaces

[shawnbot]

This PR updates to Lerna 2.1.1, uses Yarn for faster and more reliable dependency management, and uses Yarn's experimental workspaces feature to symlink dependencies during lerna bootstrap.

Bootstrapping from a clean install takes <30s on my little MacBook Air with this change! 🎉

TODO

• Make sure it works on CI
• Make sure that @broccolini and @jonrohan can run this locally without any problems

[shawnbot]

FWIW, this is an approach adopted by Jest in this PR.

[shawnbot]

Drat. I ran lerna clean and yarn clean locally, then npm test, and got the same error as on Travis.

[shawnbot]

This is too much to do in a single PR. Let's upgrade Lerna first, then try out Yarn workspaces.