Adding puppeteer should be a major version update

I support adding puppeteer. However, adding it crashed our CI process in the way, that Puppeteer requires at least Node version 6.4.0. Prior to adding Puppeteer you could use BackstopJS with at least Node version 4.8.7. This in my view represents a breaking change and according to semantic versioning the new version should be a major version update.

From the [Semantic Versioning Docs](https://semver.org/)
> MAJOR version when you make incompatible API changes,
> MINOR version when you add functionality in a backwards-compatible manner

Maybe you should consider changing the version retrospectively and republish the different versions to npm. On the other hand, I know that it probably isn't worth the hassle :D Just wanted to let you know :)

No files or tests are named. Review the Puppeteer dependency addition, the supported Node.js versions, and the published npm version history; done means deciding whether the compatibility change requires a major release and documenting or carrying out the approved release approach.

Written by the indexing model from the issue text.