update OSX Travis env before running tests

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

.travis.yml

@@ -64,6 +64,7 @@ jobs:
         # since the last OS image build (as of July 2020), but the Travis OS still
         # contains it, and it prevents updating of Python 3.
       - brew update && brew unlink python@2 && brew install gpgme
+      - sudo rm -rf /Library/Developer/CommandLineTools && sudo xcode-select --install
       script:
       - hack/travis_osx.sh
     - stage: local-build