Add Travis CI build using Xcode 11.3

Test with the latest Xcode release too.

Make this build monolithic just to try to cover more possibilities.

.travis.yml

@@ -41,6 +41,10 @@ matrix:
           compiler: clang
           env: wxTOOLSET=cmake wxCMAKE_GENERATOR=Xcode wxCMAKE_DEFINES="-DCMAKE_CXX_STANDARD=11"
           name: wxOSX Xcode 9.4
+        - os: osx
+          osx_image: xcode11.3
+          env: wxCONFIGURE_FLAGS="--enable-monolithic"
+          name: wxOSX Xcode 11.3
         - dist: trusty
           compiler: gcc
           env: wxCONFIGURE_FLAGS="--with-x11 --enable-pch --disable-stc" wxSKIP_SAMPLES=1