v6.0.0

If you are upgrading: please see UPGRADING.md.

Changed

• Breaking: bump abstract-leveldown (4e2f320, dee72a3) (Vincent Weevers)
• Breaking: drop node 6 and 8 (Level/community#98) (3708b5f) (Vincent Weevers)
• Breaking: modernize syntax and bump standard (Level/community#98) (c1f98c3) (Vincent Weevers)
• Use _nextTick() utility for browsers (ba525c4) (Vincent Weevers)
• Replace safe-buffer with buffer (d3dab23) (Vincent Weevers)
• Remove default export (Level/community#87) (5fd8368) (Vincent Weevers)
• Unlock dependencies (7126df1) (Vincent Weevers)