[ci] bump Xcode version at Travis (#3323)

* bump Xcode version at Travis

* remove temp fix for gcc at Travis

.ci/setup.sh

@@ -9,9 +9,6 @@ if [[ $OS_NAME == "macos" ]]; then
     else  # gcc
         if [[ $TASK != "mpi" ]]; then
             brew install gcc
-            if [[ $TRAVIS == "true" ]]; then
-                brew upgrade gcc
-            fi
         fi
     fi
     if [[ $TASK == "mpi" ]]; then

.travis.yml

@@ -10,7 +10,7 @@ os:
   - linux
   - osx
 dist: bionic
-osx_image: xcode11.5
+osx_image: xcode12
 
 env:
   global:  # default values