Backport the gem to Ruby 2.3.

This makes it possible to start using the gem in a project that aims to
migrate to Ruby 2.4, but must still work with 2.3 until the migration
completes.

.travis.yml

@@ -1,4 +1,6 @@
 language: ruby
 rvm:
+  - "2.3"
+  - "2.4"
   - ruby-head
 before_install: gem install bundler -v 1.11.2

test/webrick_testing.rb

@@ -1,6 +1,22 @@
 # frozen_string_literal: false
 require 'timeout'
 
+# Backport of WEBrick::Utils::TimeoutHandler.terminate from Ruby 2.4.
+unless WEBrick::Utils::TimeoutHandler.respond_to? :terminate
+  class WEBrick::Utils::TimeoutHandler
+    def self.terminate
+      instance.terminate
+    end
+
+    def terminate
+      TimeoutMutex.synchronize{
+        @timeout_info.clear
+        @watcher&.kill&.join
+      }
+    end
+  end
+end
+
 module TestXMLRPC
 module WEBrick_Testing
   def teardown

xmlrpc.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
-  spec.required_ruby_version = ">= 2.4.0dev"
+  spec.required_ruby_version = ">= 2.3"
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"