Make travis brew update more reliable

Brew update is done automatically on the first install, and timeout too many times due to the big changes between the osx image and today. travis_wait should prevent the "no output generated in the last 10 minutes"

.travis.yml

@@ -54,6 +54,7 @@ matrix:
 before_install:
   # https://github.com/travis-ci/travis-ci/issues/6307
   - if [ "${TRAVIS_OS_NAME}" == "osx" ]; then
+      travis_wait brew update
       brew install gpg2;
       brew uninstall gpg;
       gpg2 --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3;