drop node.js v6 (#2632)
drop node.js v6 (#2632)

in preparation for EOL on 2019-04-30
smfreegard/Haraka,msimerson/Haraka,msimerson/Haraka,haraka/Haraka,msimerson/Haraka,haraka/Haraka,msimerson/Haraka,haraka/Haraka,smfreegard/Haraka,smfreegard/Haraka,smfreegard/Haraka,haraka/Haraka