Remove obsolete gem SafeYAML compatibility

[marcandre]

AFAIK, there is no longer any reason to use SafeYAML.

This is what was causing issues on ruby-head, the gem is incompatible with keyword parameters.

[aav7fl]

@marcandre Would it have been better to warn instead of raising an error?

I understand the importance of letting users know they should remove that dependency if they were using RuboCop in isolation.

But I have another project (Jekyll) that I use to build my site before testing. Jekyll is pulling in SafeYAML during the build portion which then breaks my RuboCop tests.

Or should I be isolating my build/test portions and uninstalling gems between steps?

Suggested changes: #9035

[marcandre]

Let's discuss in #9035 👍