travis: Uninstall packages before upgrade

numpy (needed by cgal) started having the same issue with
linking as python, which makes upgrade and thus the entire
build fail on macOS.

Instead of playing more tricks with linking/unlinking, just
uninstall the problematic packages (and those dragging them
in) before doing anything else.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -21,10 +21,10 @@ matrix:
     - compiler: clang
       os: osx
       before_install:
+        - brew uninstall python mercurial postgis sfcgal cgal gdal
         - brew update
-        - brew unlink python
         - brew upgrade
-        - brew install rpcgen yajl
+        - brew install python rpcgen yajl
       script:
         # We can't run make distcheck/syntax-check because they
         # fail on macOS, but doing 'install' and 'dist' gives us