package/ci: some cosmic rays affecting Travis, again.

I'm using a non-beta 9.3 elsewhere without problems, so let's do that
here too.

package/ci/travis.yml

@@ -98,7 +98,9 @@ matrix:
     # PluginManager itself). The commit fixing it is from Jan 3 2018 and all
     # Xcode versions before Xcode 9.3 beta (released on Jan 24) are affected.
     # https://github.com/llvm-mirror/compiler-rt/commit/860d7953a6f36980f406bfbff76f3c00f15eed57
-    osx_image: xcode9.3beta
+    # Xcode 9.3 beta has broken git clone ("unknown protocol version 2") since
+    # Feb 16 2019, so using version 9.3 now.
+    osx_image: xcode9.3
     compiler: clang
     env:
     - JOBID=macos-gl