Ruby 2.4: "warning: constant ::Fixnum is deprecated"

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I'm getting deprecation warnings after upgrading from Ruby 2.3 to 2.4 (and reinstalling Podcatcher via gem install podcatcher):

/usr/local/lib/ruby/gems/2.4/gems/podcatcher-4.0.0/bin/podcatcher:571: warning: constant ::Fixnum is deprecated
/usr/local/lib/ruby/gems/2.4/gems/podcatcher-4.0.0/bin/podcatcher:580: warning: constant ::Fixnum is deprecated
/usr/local/lib/ruby/gems/2.4/gems/podcatcher-4.0.0/bin/podcatcher:582: warning: constant ::Fixnum is deprecated
/usr/local/lib/ruby/gems/2.4/gems/podcatcher-4.0.0/bin/podcatcher:603: warning: constant ::Fixnum is deprecated

My current environment:

% ruby --version
ruby 2.4.3p205 (2017-12-14 revision 61247) [amd64-freebsd10]
% gem --version
2.7.6
% podcatcher --version
4.0.0

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(Workaround, btw: run podcatcher as env RUBYOPT="-W0" podcatcher. Useful for suppressing the warning in cronjobs, etc.)