CI: require ruby 2.5 to always pass, and add ruby-head to versions to use
CI: require ruby 2.5 to always pass, and add ruby-head to versions to use

openSUSE/errbit,cognoa/errbit,errbit/errbit,cognoa/errbit,micred/errbit,disvelop/errbit,errbit/errbit,openSUSE/errbit,mak-it/errbit,openSUSE/errbit,mak-it/errbit,diowa/errbit,cognoa/errbit,micred/errbit,disvelop/errbit,diowa/errbit,mak-it/errbit,disvelop/errbit,errbit/errbit,micred/errbit,diowa/errbit