It fails to install homebrew.  Unfortunately we cannot mark
it as an expected failure because Travis does not match
allow_failures rows against include rows (only against the
main test matrix, which we do not use at all), so just disable
it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190220105131.23479-1-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>