Add xcode9.3 to travis and try to include it in allow_failures

.travis.yml

@@ -33,13 +33,19 @@ matrix:
       env: C_COMPILER=clang CXX_COMPILER=clang++ GENERATOR_NAME="Xcode"
       osx_image: xcode9.2
 
+    - os: osx
+      env: C_COMPILER=clang CXX_COMPILER=clang++ GENERATOR_NAME="Xcode"
+      osx_image: xcode9.3
+
   allow_failures:
     # These are additional combinations that we test but allow to fail, as they're either older compilers (g++-5) or
     # uncommon combinations that would need some workarounds or updates to work.
     - env: C_COMPILER=gcc-5 CXX_COMPILER=g++-5 GENERATOR_NAME="Unix Makefiles"
     - env: C_COMPILER=gcc-8 CXX_COMPILER=g++-8 GENERATOR_NAME="Unix Makefiles"
     - env: C_COMPILER=clang-5.0 CXX_COMPILER=clang++-5.0 GENERATOR_NAME="Unix Makefiles"
     - env: C_COMPILER=clang-6.0 CXX_COMPILER=clang++-6.0 GENERATOR_NAME="Unix Makefiles"
+    - env: C_COMPILER=clang CXX_COMPILER=clang++ GENERATOR_NAME="Xcode"
+      osx_image: xcode9.3
 
 install:
   - |