Fix remaining Minitest 6 warnings

I failed to fix this one in https://travis-ci.org/github/codegram/hyperclient/jobs/687209667

```
DEPRECATED: global use of must_equal from /home/travis/build/codegram/hyperclient/features/steps/default_config.rb:28. Use _(obj).must_equal instead. This will fail in Minitest 6.
```

Spinach + Minitest don't play along and `_(obj)` is not available.