Upgrade to the latest macOS image (#7167)

This fixes the test failures we saw last night at https://travis-ci.com/certbot/certbot/builds/116073070.

The problem is that the Homebrew installation included in the Travis image is outdated and when it tries to install packages, it fails. You can see this at https://travis-ci.com/certbot/certbot/jobs/209185570#L83. There is a thread in Travis' community froum about this at https://travis-ci.community/t/xcode-8-3-homebrew-outdated-error/3798.

To fix this, we could either upgrade Hombrew which can be a slow process according to both Travis and the original poster of the issue or we could upgrade to a newer version of macOS. I chose the latter to avoid the speed problems and picked the latest version available.

You can see tests passing with these changes at https://travis-ci.com/certbot/certbot/builds/116186095.

.travis.yml

@@ -234,6 +234,9 @@ matrix:
     - language: generic
       env: TOXENV=py27
       os: osx
+      # Using this osx_image is a workaround for
+      # https://travis-ci.community/t/xcode-8-3-homebrew-outdated-error/3798.
+      osx_image: xcode10.2
       addons:
         homebrew:
           packages:
@@ -243,6 +246,9 @@ matrix:
     - language: generic
       env: TOXENV=py3
       os: osx
+      # Using this osx_image is a workaround for
+      # https://travis-ci.community/t/xcode-8-3-homebrew-outdated-error/3798.
+      osx_image: xcode10.2
       addons:
         homebrew:
           packages: