ci(osx): use new location of the perforce cask

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.

Preface the "brew install caskroom/cask/perforce" with the old
way of installing perforce, and only try this method if the
"brew install perforce" fails.

The existing way to use caskroom was added in 672f51c (travis-ci:
fix Perforce install on macOS, 2017-01-22) and the justification
is that the "brew install perforce" can fail due to a hash
mis-match. The mismatch is due to the official Perforce distro
updating the published binaries without updating the version
string. CI servers are typically fresh virtual machines, so that
issue should not arise in automated builds.

Even if a build server is re-used and hits the hash mis-match,
it will fall back to the "new" mechanism which is currently
failing, but may be fixed independently of this change.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>

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)