Fix unicode problems in JSON encoding, fixes #4.

Not sure why json_encode() outright broke with a string like 日本語, it should have just returned escaped unicode like \u<something>. But telling json_encode() to not escape unicode and leave as-is seems to fix the problem so ¯\_(ツ)_/¯

src/Maknz/Slack/Client.php

@@ -313,7 +313,7 @@ public function clearAttachments()
    */
   public function send($message)
   {
-    $payload = json_encode($this->preparePayload($message));
+    $payload = json_encode($this->preparePayload($message), JSON_UNESCAPED_UNICODE);
 
     $this->guzzle->post($this->endpoint, ['body' => $payload]);
   }