Build with a newer version of XCode.

Travis CI by default uses XCode 7.3 on Mac OS X, which does not support
the C++11 thread_local keyword expected by Subzero. Request XCode 8 GM
instead.

Change-Id: I0afa7804997ff5b5a6bbfe7829ba6134fe3d54ca
Reviewed-on: https://swiftshader-review.googlesource.com/11548
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/.travis.yml b/.travis.yml
index d2a9c71..d949cfb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,7 @@
   - osx
 
 dist: trusty
+osx_image: xcode8
 sudo: false
 
 compiler: