Update OSX environment to Sierra

as homebrew seems to have dropped support for El Capitan in their gcc packages

Author: martin-frbg

.travis.yml

@@ -149,7 +149,7 @@ matrix:
 
     - &test-macos
       os: osx
-      osx_image: xcode8
+      osx_image: xcode8.3
       before_script:
         - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=NEHALEM NUM_THREADS=32"
         - brew update