Clarify that ruby-install does not support installing from rolling releases.
Clarify that ruby-install does not support installing from rolling releases.

* This is due to ruby-install using ruby-versions to verify the checksums of a
  downloaded release. Since rolling releases are constantly being updated,
  oftenly daily, it would be very difficult to maintain an up-to-date list of
  checksums to verify against.
* It is a better long-term strategy, for both ruby-install and end-users,
  to encourage Ruby implementations to release stable versions and to
  support installing those.

postmodern/ruby-install,postmodern/ruby-install