Running migration with umzug has some issues

When running the following migration

```js
'use strict';

module.exports = {
up: (query, types) => {
query.addColumn('Users', 'lastName', types.STRING);
},
down: (query, types) => {
query.removeColumn('Users', 'lastName');
},
};
```
This error is thrown `SequelizeDatabaseError: database "public" does not exist`
the query that it attempted to ran was this:
```
Executing (default): ALTER TABLE "public"."Users" ADD COLUMN "lastName" VARCHAR(255);
```

After digging into the code a little bit I think i tracked this down to this line here in sequelize
https://github.com/sequelize/sequelize/blob/master/lib/dialects/abstract/query-generator.js#L26

You can see they call this function in the postgres query-generator dialect on this line
https://github.com/sequelize/sequelize/blob/master/lib/dialects/postgres/query-generator.js#L241

Versions:
Cockroachdb: CCL v1.1.0-rc.1 @ 2017/10/05 15:33:25 (go1.8.3)
sequelize-cockroachdb: 1.0.2
umzug: 2.0.1