Merge pull request #210: ci(osx): use new location of the perforce cask

This fix is a rebase of gitgitgadget#400.

The CI builds are failing for Mac OS X due to a change in the
location of the perforce cask. The command outputs the following
error:

    + brew install caskroom/cask/perforce
    Error: caskroom/cask was moved. Tap homebrew/cask-cask instead.

So let's try to call `brew cask install perforce` first (which is what
that error message suggests, in a most round-about way).

The "caskroom" way was added in 672f51c (travis-ci:
fix Perforce install on macOS, 2017-01-22) and the justification
is that the call "brew cask install perforce" can fail due to a checksum
mismatch: the recipe simply downloads the official Perforce distro, and
whenever that is updated, the recipe needs to be updated, too.

CI servers are typically fresh virtual machines, but not always. To
accommodate for that, let's try harder if `brew cask install perforce`
fails, by specifically pulling the latest `master` of the
`homebrew-cask` repository.

This will still fail, of course, when `homebrew-cask` falls behind
Perforce's release schedule. But once it is updated, we can now simply
re-run the failed jobs and they will pick up that update.

As for updating `homebrew-cask`: the beginnings of automating this in
https://dev.azure.com/gitgitgadget/git/_build?definitionId=11&_a=summary
will be finished once the next Perforce upgrade comes around.

Author: derrickstolee

ci/install-dependencies.sh

@@ -40,6 +40,11 @@ osx-clang|osx-gcc)
 	test -z "$BREW_INSTALL_PACKAGES" ||
 	brew install $BREW_INSTALL_PACKAGES
 	brew link --force gettext
+	brew cask install perforce || {
+		# Update the definitions and try again
+		git -C "$(brew --repository)"/Library/Taps/homebrew/homebrew-cask pull &&
+		brew cask install perforce
+	} ||
 	brew install caskroom/cask/perforce
 	case "$jobname" in
 	osx-gcc)