Java-SDK - 2.0.0 Beta 4

# 2.0.0 Beta 4
February 1, 2018

This release is a fourth beta release supporting feature flags and rollouts. It includes all the same new features and breaking as the last beta release. It also includes the latest Notification Listeners and bucketing Id changes from 1.9.0.

## New Features
* New NotificationCenter for track and activate notifications.
* Use $opt_bucketing_id in your attribute list to support bucketing like users.