fix(deps): update dependency next-offline to v4

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
next-offline	dependencies	major	3.4.0 -> 4.0.0	source

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Release Notes

hanford/next-offline

### [`v4.0.0`](https://togithub.com/hanford/next-offline/blob/master/CHANGELOG.md#v400-2019-05-13) [Compare Source](https://togithub.com/hanford/next-offline/compare/v3.4.0...f563d3a49f735e8365dd98674a89e2675dd550a8) - Upgrade workbox-webpack-plugin from 3.6.3 to 4.3.1

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