Remove istanbul and use nyc

[adam2k]

Removing istanbul and adding nyc because istanbul is deprecated. Their recommendation is to use nyc instead.

Reference: https://www.npmjs.com/package/istanbul

[pratikpc]

@adam2k you will need to downgrade NYC to v14 as they dropped Node 6 support in v15

Learnt it the hard way

[pratikpc]

@adam2k I added an nyc switch on https://github.com/kr1sp1n/node-vault/pull/195

[aviadhahami]

Closing in favor of #195