[PATCH-for-5.0] .travis.yml: Build OSX 10.14 with Xcode 10.0

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Travis recently made a change which generates various warnings
such [*]:

    CC      utils.o
  In file included from cs.c:11:
  In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/stdio.h:64:
  /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/_stdio.h:93:16: warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Wnullability-completeness]
          unsigned char   *_base;
                          ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/_stdio.h:93:16: note: insert '_Nullable' if the pointer may be null
          unsigned char   *_base;
                          ^
                            _Nullable

We only aim to support MacOS 10.14 and 10.15. 10.14 comes with
Xcode 10.0. These warnings are not emitted with this Xcode version,
so switch back to it.

[*] https://travis-ci.org/github/qemu/qemu/jobs/673000302#L1387

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 .travis.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 2fd63eceaa..7c92206ea3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -272,12 +272,12 @@ jobs:
 
     # MacOSX builds - cirrus.yml also tests some MacOS builds including latest Xcode
 
-    - name: "OSX Xcode 10.3"
+    - name: "OSX 10.14 (Xcode 10.0)"
       env:
         - BASE_CONFIG="--disable-docs --enable-tools"
         - CONFIG="--target-list=i386-softmmu,ppc-softmmu,ppc64-softmmu,m68k-softmmu,x86_64-softmmu"
       os: osx
-      osx_image: xcode10.3
+      osx_image: xcode10
       compiler: clang
       addons:
         homebrew:
-- 
2.21.1