Prepare 4.6 release

Prepare 4.6 release

For compatibility with the yearly Christmas release of Ruby (this time
v2.6). Note I dropped the (unused anyway) ZenTest dependency, which
otherwise complains:

    ZenTest-4.11.0 requires rubygems version < 3.0, which is
    incompatible with the current version, 3.0.1

Updated local Ruby install first with:

    ruby-install ruby 2.6.0

For some reason, chruby doesn't pick it up (probably because I didn't
start a new shell), so I:

    export PATH=~/.rubies/ruby-2.6.0/bin:$PATH