Revert "pin a dependency on gherkin; don't let it go over 2.11.5 as that breaks JSON gem dependencies with Chef and a few other gems."

This reverts commit 891b2ea849018273be3924a35619d93f4636926e.

Now that Chef is releasing updated versions of its gems (10.20.0 and 11.2.0), the gherkin requirement can be removed.