rbenv-install: show the short list by default

As suggested by @eregon, now we have "-l/--list" option for the short
list of versions. Behaviour of `ruby-build --definitions` is kept as
before for compatibility.

rbenv-install:
  -L/--list-all       full list
  -l/--list           short list

ruby-build:
  -d/--definitions    full list
  -l/--list           short list