Update changelog in preparation for release

Author: Stephen Powis

CHANGELOG.md

@@ -2,7 +2,7 @@
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
-## 3.1.0 (UNRELEASED)
+## 3.1.0 (12/13/2018)
 - Officially support Kafka 2.0.x
 - KafkaTestUtils.produceRecords() and its variants now set producer configuration "acks" to "all"
 - Update Kafka-JUnit5 junit dependency from 5.1.1 to JUnit 5.3.2