ci: add Xcode 9.0 to Travis-CI build matrix

Change-Id: I57cb563e442d81162f23dfb2561099851d668637
diff --git a/.travis.yml b/.travis.yml
index 276a626..bdc17a1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -131,6 +131,7 @@
       env:
         - CXX=clang++-5.0
         - NODE_LABELS="Linux Ubuntu Ubuntu-14.04"
+        - BUILD_WITH_ASAN=yes
 
     # macOS/clang
     # https://docs.travis-ci.com/user/osx-ci-environment/#OS-X-Version
@@ -148,6 +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