The underlying `sw-precache` library has an [`engines` requirement](https://github.com/GoogleChrome/sw-precache/blob/26bbf507379f8c740b0e18b91843f00bfcfc2506/package.json#L21) of node 4+. I noticed that your library requires node 5+ (see https://github.com/facebookincubator/create-react-app/pull/1728#issuecomment-284790918). Are you using anything specific to node 5 that won't work in node 4? If no, could you lower the `engines` requirement to support node 4 as well?