Use cross-env to enable windows build

[acailly]

Setting env vars like this NODE_ENV=production doesn't works on Windows.

Using cross-env fixes this issue: cross-env NODE_ENV=production

Note that I didn't update yarn.lock file because I'm not able to do a full install due to canvas dependency build failure

[acailly]

To make it work, I put canvas as an optional dependency and added canvas -prebuilt dependency
If the tests don't find canvas, they will now use canvas-prebuilt