Let Travis build with ruby 2.6.0

[sschiffer]

Travis now uses ruby 2.6.0 via rvm for the build environment

[codecov-io]

Codecov Report

Merging #501 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   98.28%   98.29%   +<.01%     
==========================================
  Files          27       27              
  Lines        1811     1817       +6     
==========================================
+ Hits         1780     1786       +6     
  Misses         31       31

Impacted Files	Coverage Δ
lib/html-proofer/runner.rb	99.03% <0%> (ø)	⬆️
lib/html-proofer/cache.rb	92.22% <0%> (+0.08%)	⬆️
lib/html-proofer/configuration.rb	82.35% <0%> (+5.42%)	⬆️

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[gjtorikian]

Thanks!