Update Knex dep to allow 0.20.x

[mastermatt]

https://github.com/knex/knex/blob/master/CHANGELOG.md#0200---25-october-2019

Includes a small lint fix and updates package.json>engines>node to specify 8+.
I looks like Bookshelf already dropped support for Node 6 w/ 735f2ac, but this config was forgotten.

Fixes #2048

[ricardograca]

The change in minimum supported Node version is a bit troublesome since it's a breaking change. Let me think about this for a few more days.

[mastermatt]

Be aware that Knex dropped support for Node 6 back in 0.18.0.
Bookshelf adopted 0.18 in #1991 before 1.0.0 was cut, so it seems the current major version of Bookshelf already only supports 8, 10, 12, which was also reflected in the changes made in the Travis config in that same PR.
So this isn't a breaking change.

[ricardograca]

Right, that's a good point, but Bookshelf still supports older versions of Knex as you can see in peerDependencies, so it is possible to make Bookshelf work on Node.js 6 if you install the correct version of Knex.

I'll create a review to reflect these points.

[ricardograca]

We still support Node.js 6 technically, although not very actively.

[ricardograca]

Can you remove this change? It's not correct since it's still possible to run on Node 6 if you install the correct version of Knex as listed in peerDependencies.

[mastermatt]

yep. I put it back.

[ricardograca]

Looks good. Thanks for the contribution.