`ExtractPlist` doesn't require a URL like other strategies (it takes
a `Cask` instead) but we've had to provide one in existing
`livecheck` blocks to get past this requirement in livecheck. This
commit makes it so this requirement is only enforced when a
strategy's `#find_versions` method has a required `url` parameter
(not an optional one, as in `ExtractPlist`).
2194f6e12b