Babel, Clustering for Groups

• Bottleneck v2 now ships code that is compatible with Node v6 instead of requiring Node v7.6. Thanks Babel.
• Groups now support Clustering. Each limiter is automatically garbage collected, including its Redis data and even in the cases where Node loses connection to Redis.
• Returning a promise to Bottleneck from within an event listener on a limiter now ensures your promise gets fully executed before exiting your program.
• The depleted event now passes an empty argument.