travis-ci: add brew cache, remove ccache

travis-ci: add brew cache, remove ccache

1-2 minutes speedup osx builds by using brew cache.
Also, ccache was removed for a while (builds fail
after travis-ci upgraded clang to version 5.0.0)
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20180104193710.23778-1-chipitsine@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16154.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0c1b9864184e609b68c2bb317ee57317ae12a026)