Use guard

```bash
$ guard
14:33:48 - INFO - Guard is using TerminalTitle to send notifications.
14:33:48 - INFO - Guard::Minitest 2.3.1 is running, with Minitest::Unit 5.3.5!
14:33:48 - INFO - Running: all tests
Run options: --seed 56407

# Running:

.

Finished in 0.000888s, 1126.1261 runs/s, 4504.5045 assertions/s.

1 runs, 4 assertions, 0 failures, 0 errors, 0 skips

14:33:49 - INFO - Guard is now watching at '/Users/jonmagic/Projects/four_stroke'
[1] guard(main)>
```