use a newer xcode image (which means testing on sierra) (#635)
This also gets us a more up-to-date homebrew so it doesn't have to do a
massive migration.
diff --git a/.travis.yml b/.travis.yml
index f37148c..70bafca 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,11 +14,11 @@
include:
- language: generic
os: osx
- osx_image: xcode8
+ osx_image: xcode8.3
env: TOXENV=py27
- language: generic
os: osx
- osx_image: xcode8
+ osx_image: xcode8.3
env: TOXENV=py27 OPENSSL=1.1.0
- python: "2.6" # these are just to make travis's UI a bit prettier
env: TOXENV=py26