the short list now shows only latest stable versions

for each Ruby implementations like the following.

    2.4.9
    2.5.7
    2.6.5
    2.7.0
    jruby-9.2.9.0
    maglev-1.0.0
    mruby-2.1.0
    rbx-3.107
    truffleruby-19.3.1