[travis] Use newer macOS

achilleas-k · achilleas-k · commit b3f76ed02e49 · 2020-12-07T13:36:27.000+01:00
Default macOS version on Travis (10.13 High Sierra) causes issues with
homebrew.
Setting the osx_image to xcode12.2 uses macOS 10.15 (Catalina).
diff --git a/.travis.yml b/.travis.yml
@@ -19,6 +19,7 @@ matrix:
       os: linux
     - go: "1.14.x"
       os: osx
+      osx_image: xcode12.2
       addons:
         homebrew:
           packages:
