Merge pull request #3834 from ThomasWaldmann/fix-travis-osx-1.0

travis OS X: use xcode 8.3 (not broken)

.travis.yml

@@ -26,15 +26,15 @@ matrix:
           env: TOXENV=flake8
         - 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: