Support step versioning

To use selenium tests for upstream releases a s well as LTS releases, it is important to consider differences between them. Namely, regression tests for recently fixed bugs will break tests for old releases while they are important part of the test suit for the new ones. Moreover, some step might be jenkins version dependent to work properly ('I set _n_ build to keep' is one such step).

Features as well as scenarios can be decorated using `@since()` annotation accepting version as it's only argument. Such a feature/scenario will be executed only if the version is newer and skipped otherwise.

`@runner` now exposes `jenkins_version` property holding 'Gem::Version' representing the version of Jenkins under test. Step definitions and page objects may take advantage of that.

This can be easily extended to cover plugin versions as well in the future.