bring arraydumper in line with json schema spec and packagist

src/Composer/Package/Dumper/ArrayDumper.php

@@ -83,7 +83,7 @@ class ArrayDumper
         }
 
         if ($package->getReleaseDate()) {
-            $data['time'] = $package->getReleaseDate()->format('Y-m-d H:i:s');
+            $data['time'] = $package->getReleaseDate()->format(DATE_RFC3339);
         }
 
         $data = $this->dumpValues($package, $keys, $data);