Environment

(Note: This issue was detected on Travis CI and has has not been tested elsewhere.)

• JRuby version (jruby -v): 9.2.6.0
• Operating system and platform (e.g. uname -a): Ubuntu 14.04.5 LTS

Other relevant info you may wish to add:

• Travis Build log: https://travis-ci.org/jekyll/jekyll/jobs/492066402

Expected Behavior

# sample script

exp = ArgumentError.new('lorem ipsum dolor sit')
warn exp

# (On MRI):
# => "lorem ipsum dolor sit"

Actual Behavior

TypeError: no implicit conversion of ArgumentError into String