Upgrade knex dependency - test restructure

[ErisDS]

Knex is pinned to 0.8.6 and used inside of this repo purely for testing that the "knexify" adapter is able to convert from GQL to a query builder object and ultimately to the correct SQL.

The tests in this library rely on being able to do require('knex')({}), passing no config, and getting a fallback sql statement that reflects SQLite3, which used to be Ghost's default.

However, in later versions of knex, it is now required to pass in a client, e.g. require('knex')({client: 'sqlite3'}) or require('knex')({client: 'mysql'}).

This means that the tests fail after upgrading.

Using sqlite3 should make the tests work again, but that seems wrong as MySQL is now the default in Ghost. Therefore it seems like there needs to be some restructuring of the tests to make sure that we get the correct output for MySQL and SQLite3.

This is therefore a slightly bigger task than all the other dependency upgrades.

[kirrg001]

GQL was replaced by NQL. GQL is deprecated now.

The knex handling is part of mongo-knex, which is used by NQL. The knex version is no longer pinned to 0.8.6. Closing.