Multi Database tests

[nettofarah]

We currently run all of our tests using SQLite in memory.

While this runs super fast and works great it doesn't really test Polo against some specific scenarios such as how different databases quote fields or how on_duplicate works with different databases.

My assumption is that we could set up some extra databases with appraisal and get some more coverage.

[nettofarah]

I like how activerecord-import solves this problem.

We might be able to do something similar to this:
https://github.com/zdennis/activerecord-import/tree/master/test

[nettofarah]

Another option is to set up multiple databases in travis.

[nettofarah]

An even cooler option would be to setup some docker containers.