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.
Merge pull request #100 from Lytro/loosen_gherkin_requirement

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."