Xcode 11.1 for macOS 10.12 build, Xcode 9 via cron

The Xcode 9 builds often fail because of some nasty interaction between
it and the Travis build cache -- but not caching means they always take
forever because of homebrew updates.  This commit changes the build to
use a new Xcode but with a deployment target of 10.12 instead.

This means that macOS version macros for versions later than 10.12 are
available and that the tests run on a later version of macOS as well.
Therefore, it doesn't prove that a "real" Xcode 9 / macOS 10.12 build
will work.

ROS2 relies on macOS 10.12 so this introduces a (small) risk of build
failures over there.  To mitigate that, a pure Xcode 9 build is included
in the cron job.

Signed-off-by: Erik Boasson <eb@ilities.com>

.travis.yml

@@ -86,10 +86,6 @@ osx_xcode: &osx_xcode
 osx_xcode9: &osx_xcode9
   <<: *osx_xcode
   osx_image: xcode9
-  cache:
-    directories:
-      - $HOME/Library/Caches/Homebrew
-      - /usr/local/Homebrew
   addons:
     homebrew:
       packages: [ python3 ]
@@ -99,11 +95,6 @@ osx_xcode9: &osx_xcode9
       # used to ensure Homebrew is kept up-to-date and build times are kept to
       # a minimum.
       update: true
-  before_cache:
-    - brew cleanup
-    - find /usr/local/Homebrew -type d -name .git |
-      xargs -I {} dirname {} |
-      xargs -I {} git --git-dir={}/.git --work-tree={} clean -f -d -x
 
 windows_vs2017: &windows_vs2017
   os: windows
@@ -165,6 +156,9 @@ jobs:
       env: [ ARCH=x86_64, ASAN=none, BUILD_TYPE=Release, SSL=YES, GENERATOR="Unix Makefiles" ]
     - <<: *osx_xcode9
       env: [ ARCH=x86_64, ASAN=none, BUILD_TYPE=Release, SSL=NO, GENERATOR="Unix Makefiles" ]
+      if: type = cron
+    - <<: *osx_xcode
+      env: [ ARCH=x86_64, ASAN=none, BUILD_TYPE=Release, SSL=NO, GENERATOR="Unix Makefiles", MACOSX_DEPLOYMENT_TARGET=10.12 ]
     - <<: *osx_xcode
       env: [ ARCH=x86_64, ASAN=address, BUILD_TYPE=Debug, SSL=YES, GENERATOR="Unix Makefiles" ]
     - <<: *osx_xcode