Fix method signature for PHP 8

.travis.yml

@@ -11,7 +11,7 @@ services:
 
 php:
   - 7.4
-  - nightly
+  - 8
 
 before_script:
   - psql -c 'DROP DATABASE IF EXISTS test;' -U postgres

src/ConnectionManager.php

@@ -59,9 +59,8 @@ final class ConnectionManager {
 	 * Prevent serialization of this object
 	 *
 	 * @throws DomainException
-	 * @return void
 	 */
-	public function __sleep(): void
+	public function __sleep()
 	{
 		throw new DomainException('No serializing of singleton');
 	}