Travis: Use the latest OS images

Use macOS 10.14.

Stay on Ubuntu Xenial until 32630 is fixed.

Part of 32631.

.travis.yml

@@ -103,7 +103,7 @@ matrix:
     - addons:
         apt:
           sources:
-            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.4.2.x-bionic main'
+            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.4.2.x-xenial main'
               key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
           packages:
             - shellcheck
@@ -111,6 +111,7 @@ matrix:
       env: TOR="0.4.2-nightly" NETWORK_FLAVOUR="basic-min"
 
     ## Test all supported python releases
+
     ## Pre-installed in Travis xenial:
     ## https://docs.travis-ci.com/user/reference/xenial/#python-support
     ## End of Life: 1 January 2020
@@ -156,14 +157,17 @@ matrix:
 ## sudo; otherwise, we would need it for getting dependencies.)
 sudo: false
 
-## (Linux only) Use the Ubuntu Xenial Linux Image
-## deb.torproject.org doesn't support Trusty any more
+## (Linux only) Use a recent Linux image (Ubuntu Xenial)
+## We can't use Bionic, because chutney has file permissions and network
+## issues (see #32630)
+## Also, pypy isn't packaged for Travis Bionic yet
 dist: xenial
 
-## (OSX only) Use the default OSX image
+## (macOS only) Use a recent macOS image
 ## See https://docs.travis-ci.com/user/reference/osx#os-x-version
-## Default is Xcode 9.4 on macOS 10.13 as of August 2018
-#osx_image: xcode9.4
+## Default is Xcode 9.4 on macOS 10.13 as of October 2019
+## Recent is Xcode 11.2 on macOS 10.14 as of October 2019
+osx_image: xcode11.2
 
 ## The TOR env var should be kept in sync with the Linux tor version below
 ## We use the basic-min network by default, to reduce load and increase