Add a port variable.

src/Go.cr

@@ -3,6 +3,7 @@ require "kemal"
 require "json"
 
 URL = "localhost"
+PORT = "3000"
 GAME_CACHE = {} of String => Go::Game
 
 def query_game(db, id) : Go::Game?

src/Go/views/game.ecr

@@ -4,7 +4,7 @@
     var node = document.getElementById('elm-root')
     var app = Elm.Main.embed(node, {
         'black' : <%= black %>,
-        'url' : "<%= "ws://" + URL + ":" + Kemal.config.port.to_s %>",
+        'url' : "<%= "ws://" + URL + ":" + PORT %>",
         'id' : "<%= id %>",
         'size' : <%= size %>
     })