The ability to link any `.app` file without specifying the full path was
removed by c244385e. This means as of version 0.18.0 of `brew-cask` the
`alfred` cask has been broken, as it raises an error due to being unable
to find `Alfred Preferences.app` (since it is not located in the root
app directory).
`md5`, `sha1`, `sha256` all take a hexdigest string, e.g:
sha1 'f645e9da45a621415a07a7492c45923b1a1fd4d4'
`no_checksum` takes no argument, and indicates there is no checksum
for this cask. This is *not recommended*, and should only be used for
casks that have no versioned downloads.
`brew cask install` will complain if there is no sum provided (unless
`no_checksum` has been invoked), or if the sums do not match. It will
provide the computed checksum so the cask can be easily amended.
Adapted from @passcod's work in 82cc199ae6
Roland Walker