Fix PHPUnit 8 BC break by conditionally call Constraint constructor (coduo#148)

Author: nclsHart

src/PHPUnit/PHPMatcherConstraint.php

@@ -16,7 +16,9 @@ final class PHPMatcherConstraint extends Constraint
 
     public function __construct(string $pattern)
     {
-        parent::__construct();
+        if (\method_exists(Constraint::class, '__construct')) {
+            parent::__construct();
+        }
 
         $this->pattern = $pattern;
         $this->matcher = $this->createMatcher();