Convert specs from "should" to "assert" syntax

This may seem like a useless waste of time, but the "should" syntax is deprecated anyway and I personally prefer using the assert syntax anyway. This also means it'll be a lot easier to move to minitest in the future if we want to.

Don't worry, I still love rspec's runner WAY more so we'll be sticking with it for at least a little while longer ;)

No huge rush on merging this, but it will get harder and harder to do if this branch and master get diverged too much. Especially if tests are being updated often in master.