Add travis_retry to mitigate impact of GDTCCT flakes (#4119)

Author: paulb777

.travis.yml

@@ -483,9 +483,9 @@ jobs:
       env:
         - PROJECT=GoogleDataTransportCCTSupport METHOD=pod-lib-lint
       script:
-        - ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=ios
-        - ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=macos
-        - ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=tvos
+        - travis_retry ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=ios
+        - travis_retry ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=macos
+        - travis_retry ./scripts/if_changed.sh ./scripts/pod_lib_lint.rb GoogleDataTransportCCTSupport.podspec --platforms=tvos
 
     - stage: test
       osx_image: xcode10.1