ci: Update version of macOS slave for Travis-CI

Change-Id: I70908ca871ea258989ec4bf0acc8e42ea224c219
diff --git a/.travis.yml b/.travis.yml
index 49058cc..508b081 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,10 +16,10 @@
         - CXX=clang++
         - NODE_LABELS="Linux Ubuntu Ubuntu-14.04"
     - os: osx
-      osx_image: xcode7.3
+      osx_image: xcode8.2
       env:
         - CXX=clang++
-        - NODE_LABELS="OSX OSX-10.11"
+        - NODE_LABELS="OSX OSX-10.12"
 notifications:
   email:
     on_success: always