webrick: remove concurrent-ruby dev dependency

Using the Queue class in stdlib is sufficient for this test,
so there's no need for a new development dependency.

And one big reason I like webrick is it's bundled with
Ruby and has no 3rd-party C ext dependencies; so having
to download and install one is a bummer.

rack.gemspec

@@ -30,6 +30,5 @@ EOF
 
   s.add_development_dependency 'minitest', "~> 5.0"
   s.add_development_dependency 'minitest-sprint'
-  s.add_development_dependency 'concurrent-ruby'
   s.add_development_dependency 'rake'
 end

test/spec_webrick.rb

@@ -1,7 +1,6 @@
 require 'minitest/autorun'
 require 'rack/mock'
-require 'concurrent/utility/native_integer'
-require 'concurrent/atomic/count_down_latch'
+require 'thread'
 require File.expand_path('../testrequest', __FILE__)
 
 Thread.abort_on_exception = true
@@ -120,8 +119,7 @@ def is_running?
   end
 
   it "provide a .run" do
-    block_ran = false
-    latch = Concurrent::CountDownLatch.new 1
+    queue = Queue.new
 
     t = Thread.new do
       Rack::Handler::WEBrick.run(lambda {},
@@ -132,13 +130,12 @@ def is_running?
                                    :AccessLog => []}) { |server|
         block_ran = true
         assert_kind_of WEBrick::HTTPServer, server
-        @s = server
-        latch.count_down
+        queue.push(server)
       }
     end
 
-    latch.wait
-    @s.shutdown
+    server = queue.pop
+    server.shutdown
     t.join
   end