Another reference error

src/custom_modules/functions.js

@@ -9,6 +9,9 @@ module.exports = {
             console.log(message);
         }
     },
+    sleep(ms) {
+        return new Promise(resolve => setTimeout(resolve, ms));
+    },
     gpio: {
         // Boot up sanity check during debug mode
         async debugInit() {
@@ -29,7 +32,7 @@ module.exports = {
                             module.exports.log(`Toggled ${pin.key}`);
                         });
                         // Wait 1000ms before toggling again.
-                        await sleep(1000);
+                        await module.exports.sleep(1000);
                         gpio.togglePin(pin.board, err => {
                             if (err) throw err;
                             module.exports.log(`Toggled ${pin.key}`);