Allow failures on ruby-head

[ShockwaveNN]

Currently it's failing because of:
rubygems/bundler#7248

[codecov-io]

Codecov Report

Merging #576 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
- Coverage   97.49%   97.49%   -0.01%     
==========================================
  Files         567      567              
  Lines        8994     8992       -2     
==========================================
- Hits         8769     8767       -2     
  Misses        225      225

Impacted Files	Coverage Δ
...ib/ooxml_parser/common_parser/common_data/color.rb	80.24% <0%> (-0.13%)	⬇️
.../drawing/picture/chart/chart_pivot_formats_spec.rb	100% <0%> (ø)	⬆️

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