travis-ci: ask homebrew for its path instead of hardcoding it

travis-ci: ask homebrew for its path instead of hardcoding it

The TravisCI macOS build is broken because homebrew (a macOS dependency
manager) changed its internal directory structure [1]. This is a problem
because we modify the Perforce dependencies in the homebrew repository
before installing them.

Fix it by asking homebrew for its path instead of hardcoding it.

[1] https://github.com/Homebrew/brew/commit/0a09ae30f8b6117ad699b4a0439010738989c547

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