Avoid loading rspec-parameterized to avoid warnings

for parser which is emitting:

```
warning: parser/current is loading parser/ruby23, which recognizes
warning: 2.3.5-compliant syntax, but you are running 2.3.6.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
```

There's no easy way to disable this, and we're already using the
latest version. This should be harmless anyway.