Fix class name to identify client version (#362)

Class "Vonage\Versions" not found 

```php
return Versions::getVersion('vonage/client-core');
```

src/Client.php

@@ -588,7 +588,7 @@ protected function needsKeypairAuthentication(RequestInterface $request): bool
 
     protected function getVersion(): string
     {
-        return Versions::getVersion('vonage/client-core');
+        return InstalledVersions::getVersion('vonage/client-core');
     }
 
     public function getLogger(): ?LoggerInterface