HomebrewFormula: Specify version

Homebrew appeared to get confused when trying to get the version from
the URL. It looked like it was getting '64' from 'osx_amd64'. I was
getting this error when trying to install the formula:

    $ brew upgrade teddywing/passextract/passextract
    ==> Upgrading 1 outdated package, with result:
    teddywing/passextract/passextract 64
    Error: Calling build.build_bottle? is disabled!
    Use build.bottle? instead.
    /usr/local/Homebrew/Library/Homebrew/cmd/upgrade.rb:121:in `upgrade_formula'

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