Bump xml-common

Author: tvdijen

composer.json

@@ -14,7 +14,7 @@
         "ext-dom": "*",
 
         "simplesamlphp/assert": "~2.0",
-        "simplesamlphp/xml-common": "~2.7"
+        "simplesamlphp/xml-common": "~2.8"
     },
     "require-dev": {
         "simplesamlphp/simplesamlphp-test-framework": "~1.11",