Fix an issue with webpack 4

[waterfoul]

Fixes the issue in #137. Note: I did not fully test everything with webpack 4 so I would not say it's "supported" but with this patch it will work

[coveralls]

Coverage decreased (-2.5%) to 96.25% when pulling 0f558d3 on waterfoul:master into 04e0f69 on JaKXz:master.

[coveralls]

Coverage decreased (-2.5%) to 96.25% when pulling 0f46e8f on waterfoul:master into 04e0f69 on JaKXz:master.

[waterfoul]

CI failed due to webpack 4 dropping support for node 4 (which is EOL in 2 months). Does this project still need to support node 4?

[teod]

this doesn't use the new webpack tappable api, but I suggest to still merge it so we can unblock webpack v4 update.

[JaKXz]

Thank you for the contribution, but I think I'm going to favour #140 over this.