`Cask::Audit#check_livecheck_version` was never updated to handle
a referenced cask in a `livecheck` block (e.g., `cask "other-cask"`).
As a result, this audit will wrongly give an error like `Version
'1.2.3' differs from '' retrieved by livecheck.` in this scenario.
This commit ensures that a referenced cask is handled by the audit
and it won't error when the referenced check provides a good version.
