Merge branch 'sg/travis-osx-brew-breakage-workaround'

The way the OSX build jobs updates its build environment used the "--quiet" option to "brew update" command, but it wasn't all that quiet to be useful. The use of the option has been replaced with an explicit redirection to the /dev/null (which incidentally would have worked around a breakage by recent updates to homebrew, which has fixed itself already). * sg/travis-osx-brew-breakage-workaround: travis-ci: make the OSX build jobs' 'brew update' more quiet