package/ci: don't call brew update to speed up the builds a bit.

It takes over a minute alone.

package/ci/travis.yml

@@ -147,7 +147,6 @@ cache:
 
 install:
 - if [ "$TRAVIS_OS_NAME" == "linux" ] && [ "$TARGET" == "desktop" ] && [ "$BUILD_DEPRECATED" != "OFF" ]; then pip install --user urllib3[secure] && pip install --user cpp-coveralls && export CMAKE_CXX_FLAGS="$CMAKE_CXX_FLAGS --coverage"; fi
-- if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update; fi
 
 - if [ "$TRAVIS_OS_NAME" == "linux" ] && [ ! "$TARGET" == "desktop-sanitizers" ]; then export CXX=g++-4.7; fi
 - if [ "$TRAVIS_OS_NAME" == "linux" ] && [ "$TARGET" == "desktop-sanitizers" ]; then export CXX=clang++-3.8; fi