Suppress compliancy warning of ruby parser while requiring

[ItsEcholot]

This fixes the following warning when running haml-lint:

warning: parser/current is loading parser/ruby31, which recognizes 3.1.3-compliant syntax, but you are running 3.1.2.
Please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

The approach has been copied from the rubocop gem (marcandre/rubocop-ast@7081f7c).

[sds]

Thanks!