Use minitest assertions instead of rspec expectations

Anyone loves the expectations instead?

I prefer the assertion style because:

- It's usually shorter
- It usually reads better (eg: `assert_equal 1, foo` vs `expect(foo).to eq(1)`
- I always have to refer to the rspec documentation for the different things you can do with expect, whereas assertions are a standard shared across different frameworks and in some cases languages too

On the other hand I do appreciate that `expect` makes it obvious what is the actual value and expected one, whereas with `assert` you have to know the convention. I eventually got over that though, because it's a really simple standard to learn and it applies everywhere.

Thoughts?

Corresponding pull in the template:
https://github.com/interagent/pliny-template/pull/149