Use the Z pattern instead of X to support OpenJDK 6

Author: Sam Wierema

api/src/main/java/com/messagebird/MessageBirdServiceImpl.java

@@ -196,7 +196,7 @@ public <P> HttpURLConnection getConnection(final String serviceUrl, final P post
             // Specifically set the date format for POST requests so scheduled
             // messages and other things relying on specific date formats don't
             // fail when sending.
-            DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssXXX");
+            DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZZZ");
             mapper.setDateFormat(df);
 
             final String json = mapper.writeValueAsString(postData);