Update dependency.

composer.json

@@ -25,7 +25,7 @@
         "pear/validate_ispn": "dev-master",
         "phing/phing": "2.16.1",
         "serialssolutions/summon": "1.2.0",
-        "symfony/yaml": "3.4.28",
+        "symfony/yaml": "3.4.29",
         "swagger-api/swagger-ui": "2.2.10",
         "vufind-org/vufindcode": "1.1.1",
         "vufind-org/vufinddate": "1.0.0",