reformat maintenance.rb, provide `else` in `if`

Per [CASK_LANGUAGE_REFERENCE.md](https://github.com/caskroom/homebrew-cask/blob/master/doc/CASK_LANGUAGE_REFERENCE.md#always-fall-through-to-the-newest-case) an `if` should have an `else`.

This Cask is an unusual case, as the vendor has a different version for every OS revision.  So, it is likely that a Yosemite user does not want to download the Mavericks version of the software, as this `else` directs.

However, we bump up against a limitation of our code here: leaving the `url` field blank is illegal in the DSL. Any command which merely *evaluated* this Cask would die under Yosemite, because the `url` field would be empty.

A better resolution would be to accept `:unknown` in the `url` field (see #5248).  We should consider accepting `:unknown` for all stanzas in anticipation of such corner cases.

cc @alexcruice