bump version to 1.15.0

Author: Alex

src/MessageBird/Client.php

@@ -14,7 +14,7 @@ class Client
     const CONVERSATIONSAPI_ENDPOINT = 'https://conversations.messagebird.com/v1';
     const VOICEAPI_ENDPOINT = 'https://voice.messagebird.com';
 
-    const CLIENT_VERSION = '1.14.1';
+    const CLIENT_VERSION = '1.15.0';
 
     /**
      * @var string