Update webpack to current version

We are still using webpack 1.x for development on Handlebars. The update means that changes to the configuration files must be done. (Tests are failing after just bumping the version)

A pull-request should be made against the 4.x branch. I'll merge to master afterwards.

Start on the 4.x branch by locating the webpack configuration files and the tests that fail after the version bump. Review the current webpack 1.x setup before updating it. Done means the configuration supports the current webpack version, the tests pass, and the pull request targets 4.x.

Written by the indexing model from the issue text.