Java-SDK - 1.6.0

- Add event tags to `track` API and include in the event payload
- Deprecates the `eventValue` parameter from the `track` method. Should use event tags to pass in event value instead
- Gracefully handle a null attributes parameter
- Gracefully handle a null/empty datafile when using the Gson parser

https://github.com/optimizely/java-sdk/releases/tag/1.6.0