Fix return type-hint (#125)

While using the PHPUnit test helper we get this error 

```
PHP Fatal error:  Declaration of Coduo\PHPMatcher\PHPUnit\PHPMatcherConstraint::additionalFailureDescription($other) must be compatible with PHPUnit\Framework\Constraint\Constraint::additionalFailureDescription($other): string in /home/vagrant/code/vendor/coduo/php-matcher/src/PHPUnit/PHPMatcherConstraint.php on line 11
2018-03-23T23:25:18+00:00 [critical] Fatal Compile Error: Declaration of Coduo\PHPMatcher\PHPUnit\PHPMatcherConstraint::additionalFailureDescription($other) must be compatible with PHPUnit\Framework\Constraint\Constraint::additionalFailureDescription($other): string
```

```
Fatal error: Declaration of Coduo\PHPMatcher\PHPUnit\PHPMatcherConstraint::additionalFailureDescription($other) must be compatible with PHPUnit\Framework\Constraint\Constraint::additionalFailureDescription($other): string in /home/vagrant/code/vendor/coduo/php-matcher/src/PHPUnit/PHPMatcherConstraint.php on line 11
```

Author: yourwebmaker

src/PHPUnit/PHPMatcherConstraint.php

@@ -27,7 +27,7 @@ public function toString() : string
         return 'matches the pattern';
     }
 
-    protected function additionalFailureDescription($other)
+    protected function additionalFailureDescription($other) : string
     {
         return $this->matcher->getError();
     }