Use TCPServer to find available port for server

This protects against the race conditions which inevitably occur
when booting multiple capybara servers (such as running specjour).
TCPServer.new() will set recently opened sockets to SOCK_WAIT status to
ensure that a certain amount of time elapses before they are reused.

lib/capybara/server.rb

@@ -83,8 +83,10 @@ class Capybara::Server
 private
 
   def find_available_port
-    @port = 9887
-    @port += 1 while is_port_open?(@port) and not is_running_on_port?(@port)
+    server = TCPServer.new('127.0.0.1', 0)
+    @port = server.addr[1]
+  ensure
+    server.close if server
   end
 
   def is_running_on_port?(tested_port)
@@ -97,18 +99,4 @@ private
     return false
   end
 
-  def is_port_open?(tested_port)
-    Timeout::timeout(1) do
-      begin
-        s = TCPSocket.new(host, tested_port)
-        s.close
-        return true
-      rescue Errno::ECONNREFUSED, Errno::EHOSTUNREACH
-        return false
-      end
-    end
-  rescue Timeout::Error
-    return false
-  end
-
 end