Fix missing import

Author: clue

src/Flow.php

@@ -5,6 +5,7 @@
 use Fhaculty\Graph\Algorithm\BaseGraph;
 use Fhaculty\Graph\Edge\Base as Edge;
 use Fhaculty\Graph\Edge\Directed as EdgeDirected;
+use Fhaculty\Graph\Exception\UnexpectedValueException;
 use Fhaculty\Graph\Graph;
 use Fhaculty\Graph\Vertex;