Fix deprecation warnings from Gherkin

Gherkin 2.11.7 deprecated[1] `TagExpression.eval` and added
`TagExpression.evaluate` to replace it. The new version causes lots of
warnings:

    [DEPRECATION] "eval" is deprecated. Please use "evaluate" instead

This commit removes the warnings by using the new `evaluate` method and
upgrade the gherking dependency to '~> 2.11.7.' where that method is
introduced.

[1]: https://github.com/cucumber/gherkin/commit/240b00a5
Add gemfile for metadata generation.
Use SSL for rubygems source.
Bump version for release.