ci: add Xcode 9.0 to Travis-CI build matrix

Change-Id: Id8ff59b9ccd1dfa5abd56db28fede02b6aae8d36
diff --git a/.travis.yml b/.travis.yml
index dd86f59..f7bd3fe 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -149,6 +149,11 @@
       env:
         - NODE_LABELS="OSX OSX-10.12"
         - BUILD_WITH_ASAN=yes
+    - os: osx
+      osx_image: xcode9
+      env:
+        - NODE_LABELS="OSX OSX-10.12"
+        - BUILD_WITH_ASAN=yes
 
 before_script:
   - ${CXX:-c++} --version