Use C++17 and STL for Xcode 11.3 Travis CI build

Note that using C++98 results in tests build failure with clang as
libc++ doesn't allow using custom std::swap() specialization for
non-const references in its iter_swap(), it really requires using rvalue
references, which are, of course, only available in C++11 and later.

.travis.yml

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