Switch coloring gems

[gjtorikian]

This avoids GPL copyright issues.

Addresses #449 (comment)

[codecov-io]

Codecov Report

Merging #519 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #519   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          28       28           
  Lines        1867     1867           
=======================================
  Hits         1836     1836           
  Misses         31       31

Impacted Files	Coverage Δ
lib/html-proofer/log.rb	91.66% <66.66%> (ø)	⬆️

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