Require macOS 10.12 for Travis CI build using Xcode 11.3

Our tests don't compile under Mac when targeting 10.9 and using C++17 as
they try to use C++17 std::uncaught_exceptions() which is only available
since 10.12 under macOS.

We could avoid the use of this function in the tests in this build
configuration, but for now just target 10.12 instead. Yet another
alternative would be to downgrade to using C++14.

.travis.yml

@@ -43,7 +43,7 @@ matrix:
           name: wxOSX Xcode 9.4
         - os: osx
           osx_image: xcode11.3
-          env: wxCONFIGURE_FLAGS="--enable-monolithic --with-cxx=17 --enable-stl"
+          env: wxCONFIGURE_FLAGS="--enable-monolithic --with-cxx=17 --with-macosx-version-min=10.12 --enable-stl"
           name: wxOSX Xcode 11.3
         - dist: bionic
           compiler: gcc