travis-ci: fix Perforce install on macOS

travis-ci: fix Perforce install on macOS

The `perforce` and `perforce-server` package were moved from brew [1][2]
to cask [3]. Teach TravisCI the new location.

Perforce updates their binaries without version bumps. That made the
brew install (legitimately!) fail due to checksum mismatches. The
workaround is not necessary anymore as Cask [4] allows to disable the
checksum test for individual formulas.

[1] https://github.com/Homebrew/homebrew-binary/commit/1394e42de04d07445f82f9512627e864ff4ca4c6
[2] https://github.com/Homebrew/homebrew-binary/commit/f8da22d6b8dbcfcfdb2dfa9ac1a5e5d8e05aac2b
[3] https://github.com/caskroom/homebrew-cask/pull/29180
[4] https://caskroom.github.io/

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>