CI: trying to fix travis on OSX (4)

Author: cyrilbouvier

.travis.yml

@@ -31,6 +31,7 @@ matrix:
         - STATIC_OR_SHARED=
         - STATIC_OR_SHARED_CMAKE=
     - os: osx
+      osx_image: xcode11
       compiler: clang
       env:
         - INTERVAL_LIB=gaol
@@ -39,6 +40,7 @@ matrix:
         - STATIC_OR_SHARED_CMAKE=
         - CXXFLAGS=-std=c++11
     - os: osx
+      osx_image: xcode11
       compiler: clang
       env:
         - INTERVAL_LIB=gaol