Add Changelog entry

Author: robbavey

CHANGELOG.md

@@ -1,3 +1,6 @@
+## #unreleased
+  - Fix: Fix failing test spec on jruby-9.3.4.0 [#81](https://github.com/logstash-plugins/logstash-output-mongodb/pull/81)
+
 ## 3.1.7
   - Fix "wrong number of arguments" error when shipping events to MongoDB (fixes #60, #64, #65) [#66](https://github.com/logstash-plugins/logstash-output-mongodb/pull/66)