this is kind of 'egg or hen' problem:

https://github.com/mkristian/psych/tree/jruby-build

can build the gem but needs a jruby WITHOUT builtin psych to run the tests ! this jruby with psych as
default needs the a gem from the above branch - started this with

https://github.com/jruby/jruby/tree/test-psych

Since recent builds jruby 9.0.0.0-SNAPSHOT (2.2.0p0) 2015-02-27 5166bff OpenJDK 64-Bit Server VM 25.31-b07 on 1.8.0_31-b13 +jit [linux-amd64]
I am getting NameError: uninitialized constant Psych::Parser, even though gem seems to be include in build, (when trying to run ruby-processing).

@monkstone could you tell me what exactly you are running. travis runs OK including psych tests.

test script:

require 'psych'

RP_CONFIG = Psych.load_file("/home/tux/.rp5rc")
p RP_CONFIG

where .rp5rc is

---
PROCESSING_ROOT: "/home/tux/processing-3.0a5"
JRUBY: true
X_OFF: 50
Y_OFF: 20

prints

{"PROCESSING_ROOT"=>"/home/tux/processing-3.0a5", "JRUBY"=>true, "X_OFF"=>50, "Y_OFF"=>20} 

with jruby-1.7.18
and following error with jruby-9000

NameError: uninitialized constant Psych::Parser
  const_missing at org/jruby/RubyModule.java:2937
         (root) at /home/tux/.gem/ruby/2.2.0/gems/psych-2.0.13/lib/psych/nodes/stream.rb:12
        require at org/jruby/RubyKernel.java:966
         (root) at /home/tux/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1
        require at /home/tux/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:54
        require at org/jruby/RubyKernel.java:966
         (root) at /home/tux/.gem/ruby/2.2.0/gems/psych-2.0.13/lib/psych/nodes.rb:2
         (root) at /home/tux/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1
        require at org/jruby/RubyKernel.java:966
        require at /home/tux/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:54
     __script__ at yaml_test.rb:1

Now that's interesting if I remove my installed version of psych, it then works OK.

SUMMARY

So jruby-1.7.18 can cope with an installed psych whereas jruby-9000 apparently not. Perhaps other people won't see this as they use rvm, whereas I prefer a common gem home, also I don't do rails or use bundler. However could be an edge case for someone else?
Update also get same behaviour with jruby-complete
jruby 9.0.0.0-SNAPSHOT (2.2.1) 2015-03-03 626d862 OpenJDK 64-Bit Server VM 25.31-b07 on 1.8.0_31-b13 +jit [linux-amd64] ie won't play nicely with an installed version of psych, remove it works OK.
@mkristian

sorry for the late reply:

psych has native extension and you are using mri gems with native extension with jruby.

default gems will be ignored if there is regular gem is installed, i.e. jruby-9k starts using your psych-2.0.13 gem which you installed with MRI and this one does not work with JRuby.

a common GEM_HOME is OK, but at least separate it between MRI and JRuby.

after a second thought - there is nothing we can do from the jruby side of things.