Updates Webpack to version 4 and fixes the unit tests.
Sets the engine to minimal version of 6.11.5, the same on Webpack 4.
Remove Node 4 from TravisCI settings
This PR keep Webpack as a project dependency since the webpack documentation says if you use this module you don't need to install webpack as a project dependency:
https://webpack.js.org/guides/integrations/#gulp
This PR keep Webpack as a project dependency since the webpack documentation says if you use this module you don't need to install webpack as a project dependency: https://webpack.js.org/guides/integrations/#gulp