try updating homebrew

.travis.yml

@@ -670,7 +670,8 @@ before_install: |
       echo $ENABLE_HEADLESS > headless.enabled
 
       if [ -n "$IS_OSX" ]; then
-        echo "homebrew"
+        export HOMEBREW_FORCE_VENDOR_RUBY=1
+        brew upgrade
         TAPS="$(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

@@ -89,12 +89,9 @@ function pre_build {
 
     local CACHE_STAGE; (echo "$TRAVIS_BUILD_STAGE_NAME" | grep -qiF "final") || CACHE_STAGE=1
 
-    export HOMEBREW_FORCE_VENDOR_RUBY=1
-    brew style
-    brew update
-
     #after the cache stage, all bottles and Homebrew metadata should be already cached locally
     if [ -n "$CACHE_STAGE" ]; then
+        brew update
         generate_ffmpeg_formula
         brew_add_local_bottles
     fi