Update osx image

.travis.yml

@@ -33,7 +33,7 @@ matrix:
       env: DISTRIB="ubuntu" TOX_PYTHON_VERSION="py36" COVERAGE="false" LINT="true"
     - name: "Python 3.7.2 on macOS"
       os: osx
-      osx_image: xcode10.2  # Python 3.7.2 running on macOS 10.14.3
+      osx_image: xcode11  # Python 3.7.2 running on macOS 10.14.3
       language: shell       # 'language: python' is an error on Travis CI macOS
       before_install:
        - brew update && brew upgrade python