Add xcode8 to the Travis matrix

This will facilitate future macOS Sierra builds[1]

[1] https://blog.travis-ci.com/2016-09-15-new-default-osx-image-coming/

Author: ignisf

.travis.yml

@@ -6,6 +6,9 @@ rvm:
   - rbx
 matrix:
   include:
+    - rvm: 2.2
+      os: osx
+      osx_image: xcode8
     - rvm: 2.2
       os: osx
       osx_image: xcode7.3
@@ -15,6 +18,9 @@ matrix:
     - rvm: system
       os: osx
       osx_image: xcode7.3
+    - rvm: system
+      os: osx
+      osx_image: xcode8
     - rvm: ruby-2.3.1-clang
       env: CXX=clang++
     - rvm: 1.9.3