Running Ruby 2.3.0 gave me this.
/Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor/safe_yaml/lib/safe_yaml/syck_node_monkeypatch.rb:42:in `<top (required)>': uninitialized constant Syck (NameError)
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor/safe_yaml/lib/safe_yaml.rb:197:in `<module:YAML>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor/safe_yaml/lib/safe_yaml.rb:132:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor/require_vendored.rb:4:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor.rb:40:in `require_libs'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet/vendor.rb:53:in `load_vendored'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet.rb:174:in `<module:Puppet>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-3.8.5/lib/puppet.rb:29:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:127:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:127:in `rescue in require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:40:in `require'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-repl-0.0.1/lib/puppet-repl/cli.rb:1:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-repl-0.0.1/lib/puppet-repl.rb:1:in `require_relative'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-repl-0.0.1/lib/puppet-repl.rb:1:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-repl-0.0.1/bin/prepl:3:in `require_relative'
from /Users/lsc/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/puppet-repl-0.0.1/bin/prepl:3:in `<top (required)>'
from /Users/lsc/.rbenv/versions/2.3.0/bin/prepl:22:in `load'
from /Users/lsc/.rbenv/versions/2.3.0/bin/prepl:22:in `<main>'