Make symfony-assets-install use relative symlinks

This facilitates packaging, when installing in a different work PREFIX
than the final install location.

Signed-off-by: Olivier Mehani <shtrom@ssji.net>

composer.json

@@ -115,7 +115,7 @@
         "symfony-var-dir": "var",
         "symfony-web-dir": "web",
         "symfony-tests-dir": "tests",
-        "symfony-assets-install": "symlink",
+        "symfony-assets-install": "relative",
         "incenteev-parameters": {
             "file": "app/config/parameters.yml"
         }