travis.yml: bump Xcode 10 to latest dot release

According to:

  https://docs.travis-ci.com/user/reference/osx/#macos-version

we have 10.3 available so lets use it. I don't know what Apple's
deprecation policy is for Xcode because it requires an AppleID to find
out.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

.travis.yml

@@ -247,7 +247,7 @@ matrix:
     - env:
         - CONFIG="--target-list=i386-softmmu,ppc-softmmu,ppc64-softmmu,m68k-softmmu,x86_64-softmmu"
       os: osx
-      osx_image: xcode10.2
+      osx_image: xcode10.3
       compiler: clang