this was exposed in #313 by @vitorgalvao in his attempt to update the
anvil cask; by adding a link to that cask, (which just so happens to be
the cask that we chose for our "already uninstalled message" test).
the fix is simply to move the "already uninstalled" check *before* we
start to unlink.
`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 82cc199ae6bbb1e98950e71a0573ab48e6a641ee
