.travis.yml: split MacOSX builds and reduce target list

We have reached the point where the MacOSX build was regularly timing
out. So as before I've reduced the target list to "major"
architectures to try and bring the build time down. I've added an
additional MacOSX build with the latest XCode with a minimal list of
"most likely" targets on MacOS.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -107,8 +107,14 @@ matrix:
     - env: CONFIG="--disable-tcg"
            TEST_CMD=""
       compiler: gcc
-    - env: CONFIG=""
+    # MacOSX builds
+    - env: CONFIG="--target-list=aarch64-softmmu,arm-softmmu,i386-softmmu,mips-softmmu,mips64-softmmu,ppc64-softmmu,riscv64-softmmu,s390x-softmmu,x86_64-softmmu"
       os: osx
+      osx_image: xcode9.4
+      compiler: clang
+    - env: CONFIG="--target-list=i386-softmmu,ppc-softmmu,ppc64-softmmu,m68k-softmmu,x86_64-softmmu"
+      os: osx
+      osx_image: xcode10
       compiler: clang
     # Python builds
     - env: CONFIG="--target-list=x86_64-softmmu"