Require eslint 6+ as peer dependency

[bmish]

ESLint 6 has been out since 2019-06 and requiring it will enable us to use more sophisticated eslint features.

https://eslint.org/docs/user-guide/migrating-to-6.0.0

This can be merged as part of the V6 release: #114

[coveralls]

Coverage remained the same at 100.0% when pulling 0e23393 on bmish:drop-eslint-below-6 into 379df8f on platinumazure:master.