Apply prefix to foreign key's "on" attribute, too.

laravel/database/schema/grammars/grammar.php

@@ -21,7 +21,7 @@ abstract class Grammar extends \Laravel\Database\Grammar {
 		// command is being executed and the referenced table are wrapped.
 		$table = $this->wrap($table);
 
-		$on = $this->wrap($command->on);
+		$on = $this->wrap_table($command->on);
 
 		// Next we need to columnize both the command table's columns as well as
 		// the columns referenced by the foreign key. We'll cast the referenced