test 'unstable' Xcode (9beta4) ...
    OS X 10.13 (darwin16.7.0)
    Apple LLVM version 9.0.0 (clang-900.0.31)

see https://docs.travis-ci.com/user/reference/osx/#OS-X-Version

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 .travis.yml | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/.travis.yml b/.travis.yml
index cd59570062..16c1dee150 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -130,6 +130,11 @@ matrix:
       # newer stable available, OS X 10.12
       osx_image: xcode8.3
       compiler: clang
+    - env: CONFIG=""
+      os: osx
+      # unstable
+      osx_image: xcode9
+      compiler: clang
     # Plain Trusty System Build
     - env: CONFIG="--disable-linux-user"
       sudo: required
-- 
2.13.3