ci: set Travis CI Xcode version and update NODE_LABELS to 10.11

refs #3833

Change-Id: Ie6eea82b165390fbf88ab08766c25daa10c60370
diff --git a/.travis.yml b/.travis.yml
index db62c24..49058cc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,9 +16,10 @@
         - CXX=clang++
         - NODE_LABELS="Linux Ubuntu Ubuntu-14.04"
     - os: osx
+      osx_image: xcode7.3
       env:
         - CXX=clang++
-        - NODE_LABELS="OSX OSX-10.9"
+        - NODE_LABELS="OSX OSX-10.11"
 notifications:
   email:
     on_success: always