Webpack 4

[shellscape]

What kind of change does this PR introduce?

webpack version upgrade

Did you add tests for your changes?

Updated tests which needed attention.

Summary

Introduces full support for webpack v4 and removes support for lesser versions.
Upgrades several dependencies.

Does this PR introduce a breaking change?

Yes, webpack versions beneath v4 will not function with these changes.

Other information

This PR will be released as part of webpack-dev-middleware v3.

[codecov]

Codecov Report

Merging #267 into master will decrease coverage by 0.71%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
- Coverage    97.9%   97.19%   -0.72%     
==========================================
  Files           5        6       +1     
  Lines         239      214      -25     
  Branches       72        0      -72     
==========================================
- Hits          234      208      -26     
- Misses          5        6       +1

Impacted Files	Coverage Δ
lib/util.js	100% <100%> (ø)	⬆️
lib/context.js	95% <100%> (-3%)	⬇️
lib/DevMiddlewareError.js	100% <100%> (ø)
lib/middleware.js	94% <50%> (-0.12%)	⬇️
index.js	97.29% <0%> (-0.21%)	⬇️

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[followbl]

y'all are awesomesauce...thanks 🥇