Disable OS X builds on Travis

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/.travis.yml b/.travis.yml
index 8789c44..9ba98ab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -32,12 +32,6 @@
       script:
         - tests/scripts/all.sh -k 'test_depends_*' 'build_key_exchanges'
 
-    - name: macOS
-      os: osx
-      compiler: clang
-      script:
-        - tests/scripts/all.sh -k test_default_out_of_box
-
     - name: Windows
       os: windows
       script: