ci: add clang 5.0 and Xcode 9.0 to Travis-CI build matrix

Change-Id: I64ea5cce47fa6376666d0854c1454ef3b6ce79cc

.travis.yml

@@ -92,7 +92,9 @@ matrix:
       dist: trusty
       addons:
         apt:
-          sources: llvm-toolchain-trusty-3.9
+          sources:
+            - llvm-toolchain-trusty-3.9
+            - ubuntu-toolchain-r-test
           packages: clang-3.9
       env:
         - CXX=clang++-3.9
@@ -101,11 +103,24 @@ matrix:
       dist: trusty
       addons:
         apt:
-          sources: llvm-toolchain-trusty-4.0
+          sources:
+            - llvm-toolchain-trusty-4.0
+            - ubuntu-toolchain-r-test
           packages: clang-4.0
       env:
         - CXX=clang++-4.0
         - NODE_LABELS="Linux Ubuntu Ubuntu-14.04"
+    - os: linux
+      dist: trusty
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-trusty-5.0
+            - ubuntu-toolchain-r-test
+          packages: clang-5.0
+      env:
+        - CXX=clang++-5.0
+        - NODE_LABELS="Linux Ubuntu Ubuntu-14.04"
 
     # macOS/clang
     # https://docs.travis-ci.com/user/osx-ci-environment/#OS-X-Version
@@ -121,6 +136,10 @@ matrix:
       osx_image: xcode8.3
       env:
         - NODE_LABELS="OSX OSX-10.12"
+    - os: osx
+      osx_image: xcode9
+      env:
+        - NODE_LABELS="OSX OSX-10.12"
 
 before_script:
   - ${CXX:-c++} --version