Updating to Webpack 4

[cezarsmpio]

Fixes #133

[codecov-io]

Codecov Report

Merging #141 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   80.68%   80.75%   +0.07%     
==========================================
  Files          11       11              
  Lines         264      265       +1     
  Branches       38       38              
==========================================
+ Hits          213      214       +1     
  Misses         44       44              
  Partials        7        7

Impacted Files	Coverage Δ
src/build/webpack.js	100% <100%> (ø)	⬆️

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[cezarsmpio]

Any review on this PR? :)