let's use xcode8.3 (still macOS 10.12) (#3582)

* let's use xcode8.3 (still macOS 10.12)

* update comment
diff --git a/.travis.yml b/.travis.yml
index a00b869..21a4d7a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -42,33 +42,33 @@
           env: TOXENV=py3pep8
         - language: generic
           os: osx
-          # 8.1 is macOS 10.12.x
+          # 8.3 is macOS 10.12.x
           # see: https://docs.travis-ci.com/user/osx-ci-environment/
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py27 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py33 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py34 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py35 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py36 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=pypy-nocoverage CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1 PYPY_VERSION=5.7.1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=py27 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=0
         - language: generic
           os: osx
@@ -82,7 +82,7 @@
           env: TOXENV=py27 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
-          osx_image: xcode8.1
+          osx_image: xcode8.3
           env: TOXENV=docs CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
 
 install: