Use newer version of os-x compiler.

There is a warning that xcode6.4 is going away.
diff --git a/.travis.yml b/.travis.yml
index 9d779c8..82c9ef7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
 sudo: required
 dist: trusty
 services: docker
-osx_image: xcode6.4
+osx_image: xcode8
 
 env:
   global: