Fixes #2493. Update some npm deps.

[miketaylr]

No description provided.

[miketaylr]

r? @magsout

Is there anything scary about these major version bumps related to postcss?

Locally, gruntcss is working, and tests are passing on Travis... but there might be some differences I'm not observing?

[magsout]

@miketaylr no worries, #2314 (comment)

So you can be confident with this major updated.

What I did:

git fetch origin
git checkout -b issues/2493/1 origin/issues/2493/1
rm -Rf node_modules
npm install
npm run start

No errors in terminal, and site looks good .

r + @miketaylr

I let you merge because of the label you putted

[miketaylr]

Thanks @magsout!