Replace guard with the much lighter rerun gem

[solebared]

Why

guard with guard-rspec is a fine solution but presented a few imperfections:

1. It would slow down over time; every automatically triggered run would take slightly longer.
2. By default it uses a 1 to 1 mapping of spec to app code. This is often a good choice, but i often want to run a set of specs as i make related changes. This is difficult to achieve with guard.
3. It has a non-trivial configuration scheme. This is not often a problem since plugin gems usually generate configuration for us, but when we do need to tweak something, there's a significant api to learn.

rerun is a much simpler tool that addresses all of the above.

Outstanding Questions, Concerns and Other Notes

I'm not aware of any core/recent contributors using guard, but if anyone is, we can certainly keep it alongside rerun.
In fact the latter doesn't even need to be in the project Gemfile; there's just a slight advantage to putting such tooling in here to reduce external tooling (eg in a docker container).

[maebeale]

👍