cask/cmd/info.rb: Fix crash when running brew cask info --github

[whoiswillma]

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Fix crash when using brew cask info --github sl caused by formulae being passed to Cask::Cmd::Info.github_info

Error: undefined method `sourcefile_path' for #<Formulary::FormulaNamespacee3b11e986e0540a2aeccc9b89fd2c30f::Sl:0x00007f9d0d193528>

[MikeMcQuaid]

Nice catch @whoiswillma!