try brew doctor

.travis.yml

@@ -670,8 +670,6 @@ before_install: |
       echo $ENABLE_HEADLESS > headless.enabled
 
       if [ -n "$IS_OSX" ]; then
-        export HOMEBREW_FORCE_VENDOR_RUBY=1
-        brew upgrade
         TAPS="$(brew --repository)/Library/Taps"
         if [ -e "$TAPS/caskroom/homebrew-cask" -a -e "$TAPS/homebrew/homebrew-cask" ]; then
             rm -rf "$TAPS/caskroom/homebrew-cask"

travis_config.sh

@@ -88,6 +88,7 @@ function pre_build {
     echo "Running for OSX"
 
     local CACHE_STAGE; (echo "$TRAVIS_BUILD_STAGE_NAME" | grep -qiF "final") || CACHE_STAGE=1
+    brew doctor
 
     #after the cache stage, all bottles and Homebrew metadata should be already cached locally
     if [ -n "$CACHE_STAGE" ]; then