Redundant lockfiles (npm and yarn)

[frangio]

There's already a yarn.lock file in the repository. It's not clear if we should also add npm5's package-lock.json.

See update below.

[frangio]

Keeping an eye on yarnpkg/yarn#3614. We're not the only ones confused by this. 🙂

[frangio]

There are now both yarn.lock and package-lock.json files in the repository. They are kept in sync via yarn import after #363.

[shrugs]

We should probably pick one and enforce its usage. I'm pretty ambivalent about which one the project chooses, but having both seems unnecessary. I can put out a PR for this later if we've made a decision.

[frangio]

Currently yarn is used in Travis, but most of us use npm locally. Travis was set up to use yarn in #250 because of performance reasons.

I'd be in favor of moving to npm completely and removing yarn.lock, now that npm 5 is more performant.