Ignore Jekyll redirect_from template from favicon check

[gjtorikian]

Closes #446, though it is extraordinarily brittle, there's not much else that can be done.

[codecov-io]

Codecov Report

Merging #448 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   98.52%   98.52%   +<.01%     
==========================================
  Files          27       27              
  Lines        1758     1763       +5     
==========================================
+ Hits         1732     1737       +5     
  Misses         26       26

Impacted Files	Coverage Δ
lib/html-proofer/check/favicon.rb	100% <100%> (ø)	⬆️
spec/html-proofer/favicon_spec.rb	100% <100%> (ø)	⬆️

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