Anywhere we can use `blob/master` we can use `blob/HEAD` instead. This
will make life easier if we ever rename our default branch in future
(once/if Git and GitHub provides the necessary tooling to do so).
In a number of Cask specs, the value of the `homepage` stanza is currently set
to https://example.com. As of 2018-11-28, the TLS certificate served by
example.com seems to be expired, possibly due to an oversight on ICANN’s side.
While the certificate is certainly going to be renewed soon, it would be
desirable for Homebrew’s test result to be less dependent on ICANN’s actions.
This commit changes the homepages of all test Casks to http://brew.sh, whose
domain and TLS certificate are both controlled by Homebrew.
