v6.0.0

If you are upgrading: please see UPGRADING.md.

Changed

• Breaking: modernize syntax and bump standard (Level/community#98) (4770d17) (Vincent Weevers)
• Breaking: bump levelup and encoding-down (c92e054) (Vincent Weevers)
• Add files to package.json (809fb8e) (Vincent Weevers)

Removed

• Breaking: drop node 6 and 8 (Level/community#98) (5fd4a37) (Vincent Weevers)