Fix Travis CI build.

PiperOrigin-RevId: 309138195

.travis.yml

@@ -4,7 +4,7 @@
 
 language: cpp
 dist: bionic
-osx_image: xcode10.3
+osx_image: xcode11.3
 
 compiler:
 - gcc
@@ -17,6 +17,12 @@ env:
 - BUILD_TYPE=Debug
 - BUILD_TYPE=RelWithDebInfo
 
+jobs:
+  allow_failures:
+  # Homebrew's GCC is currently broken on XCode 11.
+  - compiler: gcc
+    os: osx
+
 addons:
   apt:
     sources: