* Ensure ListenerManager listeners are synchronized

Prevents ConcurrentModificationException

* Upgrade Gson library to 2.8.2

* Bump version to 4.15.0

* Bump version to 4.15.0

* Add new checkstyle.xml file

* Update tests for 4.15.0

* Downgrade checkstyle for Java 7

* Add Javadoc for 4.15.0

* Fix a bug where TelemetryManager prevented PubNub from destroying

* Fix Codacy complaints (#95)

* Delete test (#97)

* Bugfix for yaml (#98)

* Fix yaml

* Update javadoc