rubygems not working with ruby 2.2?

[Koichiro IWAO]

Reproduced in clean environment.

I think devel/ruby-gems is too old and needs updating.
ports version is 1.8.30 and the latest rubygems release is 2.4.6[1].

[1] https://github.com/rubygems/rubygems/releases

# confirm default ruby version is set to 2.2
grep DEFAULT_VERSIONS /etc/make.conf
DEFAULT_VERSIONS=	ruby=2.2 perl5=5.20

# remove all pkgs
pkg remove -fa

# update ports tree
portsnap fetch update

# install ruby and some rubygem- ports
make -C /usr/ports/lang/ruby22
make -C /usr/ports/devel/rubygem-rake
(snip)
===>   rubygem-rake-10.4.2 depends on file: /usr/local/bin/gem22 - found
===>   rubygem-rake-10.4.2 depends on file: /usr/local/bin/ruby22 - 
found
===>  Configuring for rubygem-rake-10.4.2
===>  Building for rubygem-rake-10.4.2
/usr/local/lib/ruby/site_ruby/2.2/rubygems/specification.rb:1960:in 
`block (2 levels) in to_yaml': stack level too deep (SystemStackError)
	from /usr/local/lib/ruby/2.2/psych/coder.rb:36:in `map'
	from 
/usr/local/lib/ruby/site_ruby/2.2/rubygems/specification.rb:1959:in 
`block in to_yaml'
	from /usr/local/lib/ruby/2.2/psych/deprecated.rb:19:in `call'
	from /usr/local/lib/ruby/2.2/psych/deprecated.rb:19:in `block in 
quick_emit'
	from 
/usr/local/lib/ruby/site_ruby/2.2/rubygems/specification.rb:1960:in 
`block (2 levels) in to_yaml'
	from /usr/local/lib/ruby/2.2/psych/coder.rb:36:in `map'
	from 
/usr/local/lib/ruby/site_ruby/2.2/rubygems/specification.rb:1959:in 
`block in to_yaml'
	from /usr/local/lib/ruby/2.2/psych/deprecated.rb:19:in `call'
	 ... 9619 levels...
	from 
/usr/local/lib/ruby/site_ruby/2.2/rubygems/command_manager.rb:147:in 
`process_args'
	from 
/usr/local/lib/ruby/site_ruby/2.2/rubygems/command_manager.rb:117:in 
`run'
	from /usr/local/lib/ruby/site_ruby/2.2/rubygems/gem_runner.rb:65:in 
`run'
	from /usr/local/bin/gem22:30:in `<main>'
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure 
to
the maintainer.
*** Error code 1

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