update ruby version used by travis

https://www.ruby-lang.org/en/downloads/ says, ruby 2.2 is EOL
So I would recommend to update to 2.5 (as I am running local tests with that version and it is currently supported)

Has to be done here:
https://github.com/freifunkMUC/freifunkmuc.github.io/blob/master/.travis.yml#L3