Merge pull request #3809 from ThomasWaldmann/fix-travis-osx

travis OS X: try newer xcode versions

.travis.yml

@@ -31,15 +31,15 @@ matrix:
           env: TOXENV=py36
         - language: generic
           os: osx
-          osx_image: xcode6.4
+          osx_image: xcode8.3
           env: TOXENV=py34
         - language: generic
           os: osx
-          osx_image: xcode6.4
+          osx_image: xcode8.3
           env: TOXENV=py35
         - language: generic
           os: osx
-          osx_image: xcode6.4
+          osx_image: xcode8.3
           env: TOXENV=py36
 
 before_install: