Update macOS version on Travis CI to the latest v10.15(not v11 yet)

Reference:
- https://docs.travis-ci.com/user/reference/osx/

Author: PeterDaveHello

.travis.yml

@@ -98,6 +98,7 @@ jobs:
 # test all echo functions on difference shell(except sh, which is actually bash) on Mac
   - stage: macOS runtime test
     os: osx
+    osx_image: xcode12.2
     env:
       - task=test-dist-zsh
     before_script:
@@ -107,6 +108,7 @@ jobs:
       - ./test-scripts/zsh
   - stage: macOS runtime test
     os: osx
+    osx_image: xcode12.2
     env:
       - task=test-dist-ksh
     before_script:
@@ -116,6 +118,7 @@ jobs:
       - ./test-scripts/ksh
   - stage: macOS runtime test
     os: osx
+    osx_image: xcode12.2
     env:
       - task=test-dist-bash-builtin
     before_script:
@@ -124,6 +127,7 @@ jobs:
       - ./test-scripts/bash
   - stage: macOS runtime test
     os: osx
+    osx_image: xcode12.2
     env:
       - task=test-dist-bash-brew-install
     before_script:
@@ -134,6 +138,7 @@ jobs:
       - ./test-scripts/bash
   - stage: macOS runtime test
     os: osx
+    osx_image: xcode12.2
     env:
       - task=test-dist-fish
     before_script: