ci(travis): rollback xcode build version

BREAKING CHANGE: xcode 11.6 travis build

Author: apaparazzi0329

.travis.yml

@@ -8,19 +8,19 @@ jobs:
   - stage: Validate Pods
     script: "./Scripts/pod-lint.sh"
     os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.6
   - stage: release new version
     script: "./Scripts/travis/new-release.sh"
     os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.6
     if: branch = master AND type = push AND fork = false
   - stage: deploy to cocoapods
     script: "./Scripts/travis/deploy-to-cocoapods.sh"
     os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.6
     if: branch = master AND type = push AND fork = false
   - stage: publish documentation
     script: "./Scripts/travis/publish-documentation.sh"
     os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.6
     if: branch = master AND type = push AND fork = false