Refactor variables in rbenv::version
Refactor variables in rbenv::version

Group all variables used by the Exec[] into one place. Move the `unless`
parameter to a variable, like the others, partly so that it doesn't exceed
80chars.

Note that `$cmd_unless` must come after `$env_string`. Thankfully the spec
tests will catch empty variables.

gds-operations/puppet-rbenv,gds-operations/puppet-rbenv