Update resolver and loader syntax to work with webpack 4

[marcokam]

@amireh for some reason, I wasn't able to run the tests on the project. At first I thought it was because I'm on Windows but I also tried on OSX and it also failed. I tried these changes my work project using both webpack 3 and webpack 4 and it works for both. Let me know if any additional fixes are needed.

[codecov-io]

Codecov Report

Merging #215 into master will decrease coverage by 0.12%. The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master    #215      +/-   ##
=========================================
- Coverage   94.53%   94.4%   -0.13%     
=========================================
  Files          25      25              
  Lines         713     715       +2     
=========================================
+ Hits          674     675       +1     
- Misses         39      40       +1

Impacted Files	Coverage Δ
lib/HappyLoader.js	82.14% <50%> (ø)	:arrow_up:
lib/WebpackUtils.js	89.06% <71.42%> (-1.27%)	:arrow_down:

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[amireh]

Thanks for the patch!

Regarding the tests, you need to run npm install in every folder under upstream/ so that we can test against different versions of webpack then you run npm test:webpack{1..4} as needed.

It's a bit convoluted but works. Perhaps I'll add some instructions to the README. Anyway, I'm pulling this down for testing.

[amireh]

The tests needed a lot of rework and we still have to fix loaderContext.resolve but I'll do it locally.

Thanks again for the contribution.

[amireh]

Your change has been incorporated in the 5.0.0-beta.1 release (happypack@next) - please give it a try and leave feedback in #200 .