fix: drop node v6/7 support

[michael-siek]

BREAKING CHANGE: Drop support for NodeJS 6 and NodeJS 7

Closes issue: #133

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[stephenmathieson]

This is a breaking change. We should get @WilcoFiers to OK this first.

[schne324]

This is a breaking change

Please update the commit message body with BREAKING CHANGE: {breaking change info here}