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

refs #3833

Change-Id: I4c9f98d11e01ce1b712295c884d49fc14def37e2
diff --git a/.travis.yml b/.travis.yml
index 127eddf..1066eb7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,9 +13,10 @@
         - 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