Webpack 4 support 🚀

[pirelenito]

• Use new hooks API
• Update Node versions in Travis (Node 4 is no longer supported in Webpack 4)
• Update tests

References:

• https://medium.com/@Uneducated/how-to-write-upgrade-your-webpack-plugin-for-adding-support-for-webpack-4-9e6f1ab81677
• https://webpack.js.org/api/compilation-hooks/
• https://github.com/pedoe/docker_compose/blob/836deafe8c026518602ebb52eca8557637fb9596/jenkins/jenkins_home/.cache/yarn/v1/npm-webpack-4.0.1-768d708beeca4c5f77f6c2d38a240fb6ff50ba5d/lib/TemplatedPathPlugin.js#L102

[pirelenito]

Rejecting this in favor of #22