Bump xml-common

Author: tvdijen

composer.json

@@ -18,7 +18,7 @@
 
         "simplesamlphp/assert": "~2.0",
         "simplesamlphp/saml2": "~6.1",
-        "simplesamlphp/xml-common": "~2.7"
+        "simplesamlphp/xml-common": "~2.8"
     },
     "require-dev": {
         "simplesamlphp/simplesamlphp-test-framework": "~1.11"