Delete yarn.lock and update package-lock.json

[rsimha]

Cleans up the bad merge described in #9775 (comment)

Follow up to #11279
Fixes #11278

[rsimha]

/to @erwinmombay @danielrozenberg

[dreamofabear]

Does package-lock.json need to be updated? The Babel 6 upgrade modified package.json.

[rsimha]

@choumx Good point, but there were no changes when I blew away my node_modules directory and re-ran npm prune && npm install on my machine.

The documentation at https://docs.npmjs.com/files/package-lock.json says that the files would be changed if necessary, but I'm somehow not convinced. @erwinmombay @danielrozenberg Is this weird? Can one or both of you test this on your machines to make sure we're okay?

[rsimha]

@choumx Scratch my earlier comment. I had an older version of npm on my machine (the one that comes with node 4 by default), and it's all good once I upgraded.

I'll update the instructions if necessary.

[rsimha]

Can confirm that yarn is no longer run on Travis, and that npm install (with the correct version of npm) takes ~16 s on Travis.

Will merge this once tests are green.