Upgrade to the latest Ruby version and libraries

[paoloambrosio]

This should NOT require changing the Travis configuration. It should just work upgrading Ruby dependencies to the latest version and fixing Ruby step definitions.

Step definitions in Cucumber-TCK support an old version of Aruba. Could either fix that or copy it inside the project like the other implementations seem to have.

[paoloambrosio]

More info on #87

[konserw]

So what are we doing with cucumber-tck? It's quite crucial to decide as I've encountered more problems with it when working on CI for Windows and OSX, see #96 and #97

[paoloambrosio]

Good enough for now. Raised #115 to track further work needed.