Added Ref gem dependency on non-JRuby systems.

This commit is only viable because we create a JRuby-specific gem build.
The conditional statement in the gemspec file is not processed at
runtime. It is only processed when the gem is built. Therefore it
*should* prevent the `Ref` gem from being installed when using the
JRuby-specific build. This has not been tested, however.

Author: jdantonio

concurrent-ruby.gemspec

@@ -31,4 +31,8 @@ Gem::Specification.new do |s|
   end
 
   s.required_ruby_version = '>= 1.9.3'
+
+  unless defined?(JRUBY_VERSION)
+    s.add_dependency 'ref', '~> 1.0.5'
+  end
 end

lib/concurrent/atomic/thread_local_var.rb

@@ -8,11 +8,8 @@ module ThreadLocalRubyStorage
 
       protected
 
-      begin
+      unless RUBY_PLATFORM == 'java'
         require 'ref'
-      rescue LoadError
-        raise LoadError,
-              'ThreadLocalVar requires ref gem installed on MRI to avoid memory leaks. It\'s not concurrent-ruby dependency.'
       end
 
       def allocate_storage