[DRE-maint] Bug#892508: ruby-psych FTBFS with Ruby 2.5

[Adrian Bunk]

Source: ruby-psych
Version: 2.2.4-6
Severity: serious

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/ruby-psych.html

...
   dh_auto_configure -O--buildsystem=ruby
	dh_ruby --configure
   debian/rules override_dh_auto_build-indep
make[1]: Entering directory '/build/1st/ruby-psych-2.2.4'
jruby -S rake compile
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant SNAKEYAML_VERSION
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant ANY
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF8
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF16LE
/usr/lib/ruby/vendor_ruby/psych.rb:6: warning: already initialized constant UTF16BE
/usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:59:in `require':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
rake aborted!
LoadError: load error: psych -- java.lang.NoClassDefFoundError: Could not initialize class org.jruby.ext.psych.PsychEmitter
/build/1st/ruby-psych-2.2.4/Rakefile:1:in `(root)'
/build/1st/ruby-psych-2.2.4/Rakefile:13:in `block in (root)'
(See full trace by running task with --trace)
make[1]: *** [debian/rules:9: override_dh_auto_build-indep] Error 1