Use xcode8 on Travis to better match prod

Our new 10.11.6 macOS machines have Xcode 8.2 installed.
Use xcode8 on Travis, which maps to Xcode 8gm and macOS 10.11.

This is also important to help ensure tests like the sshd config test
match the behavior we'll see in the field.

Author: aneeshusa

.travis.yml

@@ -17,7 +17,7 @@ matrix:
         - SALT_NODE_ID=servo-mac1
         - SALT_FROM_SCRATCH=true
       os: osx
-      osx_image: xcode7.3
+      osx_image: xcode8
     - env:
         - SALT_NODE_ID=servo-linux1
         - SALT_FROM_SCRATCH=true
@@ -41,7 +41,7 @@ matrix:
         - SALT_NODE_ID=servo-mac1
         - SALT_FROM_SCRATCH=false
       os: osx
-      osx_image: xcode7.3
+      osx_image: xcode8
     - env:
         - SALT_NODE_ID=servo-linux1
         - SALT_FROM_SCRATCH=false