Fix killing webkit_server on JRuby

Connection now waits until webkit_server has started to
discover the pid.

lib/capybara/webkit/connection.rb

@@ -47,12 +47,12 @@ module Capybara::Webkit
     def start_server
       open_pipe
       discover_port
+      discover_pid
       forward_output_in_background_thread
     end
 
     def open_pipe
-      _, @pipe_stdout, @pipe_stderr, wait_thr = Open3.popen3(SERVER_PATH)
-      @pid = wait_thr[:pid]
+      _, @pipe_stdout, @pipe_stderr, @wait_thr = Open3.popen3(SERVER_PATH)
       register_shutdown_hook
     end
 
@@ -81,6 +81,10 @@ module Capybara::Webkit
       end
     end
 
+    def discover_pid
+      @pid = @wait_thr[:pid]
+    end
+
     def forward_output_in_background_thread
       Thread.new do
         Thread.current.abort_on_exception = true