diff --git a/.travis.yml b/.travis.yml
index b8690b5..3eee3d2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,9 +12,6 @@ rvm:
   - ruby-head
   - jruby-head
   - rbx-3.84
-before_install:
-  - gem update --system
-  - gem install bundler
 jdk:
   - oraclejdk8
 
diff --git a/Gemfile b/Gemfile
index 5033c41..a8b93de 100644
--- a/Gemfile
+++ b/Gemfile
@@ -11,6 +11,7 @@ group :testing do
   gem 'test-unit', '~> 3.0.9'
   gem 'rspec', '~> 3.6'
   gem 'coveralls', '~> 0.8.21', :require => false
+  gem 'json', '< 2.3', :platforms => [:ruby_19, :jruby], :require => false
 end
 
 group :documentation do
diff --git a/lib/ref.rb b/lib/ref.rb
index 1bf9e2f..cdc939d 100644
--- a/lib/ref.rb
+++ b/lib/ref.rb
@@ -5,6 +5,7 @@ module Ref
   require 'ref/abstract_reference_key_map'
   require 'ref/reference'
   require 'ref/reference_queue'
+  require 'monitor'
 
   if defined?(Java)
     begin