Revert "travis osx: run brew update first, fixes #3527"

This reverts commit e4828078bbc6ceba34a1b923b4c50a31b10b4f08.

.travis/install.sh

@@ -4,7 +4,6 @@ set -e
 set -x
 
 if [[ "$(uname -s)" == 'Darwin' ]]; then
-    brew update
     if [[ "${OPENSSL}" != "0.9.8" ]]; then
         brew outdated openssl || brew upgrade openssl
     fi