travis: Don't install xz on macOS

It's already being dragged in by Python.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

.travis.yml

@@ -18,7 +18,7 @@ matrix:
         - brew update
         - brew unlink python
         - brew upgrade
-        - brew install rpcgen yajl xz
+        - brew install rpcgen yajl
       script:
         # We can't run make distcheck/syntax-check because they
         # fail on macOS, but doing 'install' and 'dist' gives us