travis-ci: make the OSX build jobs' 'brew update' more quiet

travis-ci: make the OSX build jobs' 'brew update' more quiet

Before installing the necessary dependencies, our OSX build jobs run
'brew update --quiet'.  This is problematic for two reasons:

  - This '--quiet' flag apparently broke overnight, resulting in
    errored builds:

      +brew update --quiet
      ==> Downloading https://homebrew.bintray.com/bottles-portable-ruby/portable-ruby-2.3.7.mavericks.bottle.tar.gz
      ######################################################################## 100.0%
      ==> Pouring portable-ruby-2.3.7.mavericks.bottle.tar.gz
      Usage: brew update_report [--preinstall]
      The Ruby implementation of brew update. Never called manually.
              --preinstall                 Run in 'auto-update' mode (faster, less
                                           output).
          -f, --force                      Override warnings and enable potentially
                                           unsafe operations.
          -d, --debug                      Display any debugging information.
          -v, --verbose                    Make some output more verbose.
          -h, --help                       Show this message.
      Error: invalid option: --quiet
      The command "ci/install-dependencies.sh" failed and exited with 1 during .

    I belive that this breakage will be noticed and fixed soon-ish, so
    we could probably just wait a bit for this issue to solve itself,
    but:

  - 'brew update --quiet' wasn't really quiet in the first place, as
    it listed over about 2000 lines worth of available packages that
    we absolutely don't care about, see e.g. one of the latest
    'master' builds:

      https://travis-ci.org/git/git/jobs/486134962#L113

So drop this '--quiet' option and redirect 'brew update's standard
output to /dev/null to make it really quiet, thereby making the OSX
builds work again despite the above mentioned breakage.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>