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ARROW-4373: [Packaging] Travis fails to deploy conda packages on OSX

It fixes the osx clang crossbow builds: https://travis-ci.org/kszucs/crossbow/builds/484341387

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>

Closes #3486 from kszucs/ARROW-4373 and squashes the following commits:

760cdb3b <Krisztián Szűcs>  don't remove homebrew, because travis can't deploy without the ruby packages