Puppeteer should be a dev dependency, not an optional dependency

[ngyikp]

Installing webpack-subresource-integrity@1.1.0-rc.2 as a dependency would install Puppeteer unnecessarily and bloat node_modules size, Puppeteer should be included as a dev dependency instead.

[jscheid]

Thanks for the bug report. You're right, I'll change it and aim to release in 1.1.0-rc.3 sometime soon.

(Ideally it would be in "optional dev dependencies" but there is no such thing. npm/npm#3870 (comment))