Change from istanbul to nyc

[eddiemoore]

Closes #48

[codecov]

Codecov Report

Merging #103 into master will decrease coverage by 2.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   90.87%   87.88%   -2.99%     
==========================================
  Files          18       18              
  Lines         296      289       -7     
  Branches       69       69              
==========================================
- Hits          269      254      -15     
- Misses         27       35       +8

Impacted Files	Coverage Δ
lib/codecov.js	80% <0%> (-5.15%)	⬇️
lib/detect.js	92.3% <0%> (-0.55%)	⬇️
lib/services/gitlab.js	100% <0%> (ø)	⬆️

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