Don't use deprecated API in webpack4

[jquense]

fixes #22

Sorry did this in the GH editor!

[jankanty]

Hey. These errors looks like they are not working properly in ci: This is likely because this test is run in a continuous integration (CI) environment in which snapshots are not written by default. Can you look at it and consider accepting this PR?

[anthonyhastings]

Changes look correct to me; I'm looking forward to getting this deprecation warning out of my console!

[kylecesmat]

@jquense @jankanty @stipsan is there anything blocking this from being merged/released? It's a minor warning, but would be nice to have it removed from output :)

[stipsan]

The ci error should be fixable by running npm test — -u and pushing the updated snapshots. Although it looks like the same error is happening in master, ot doesn't look like thos PR broke any test 🤔