Fix composer 2 compatibilit issue

With Composer 1 this would already throw a warning:

```
Deprecation Notice: Class Emico\RobinHqLib\Client\InvalidApiResponseException located in ./vendor/emico/robinhq-lib/src/Client/Exception/InvalidApiResponseException.php does not comply with psr-4 autoloading standard. It will not autoload anymore in Composer v2.0. in phar:///home/rutger/bin/composer.phar/src/Composer/Autoload/ClassMapGenerator.php:201
```

When installed via Composer 2 it would say

```
Class Emico\RobinHqLib\Client\InvalidApiResponseException located in ./vendor/emico/robinhq-lib/src/Client/Exception/InvalidApiResponseException.php does not comply with psr-4 autoloading standard. Skipping.
```

src/Client/Exception/InvalidApiResponseException.php

@@ -5,11 +5,11 @@
  * @copyright (c) Emico B.V. 2017
  */
 
-namespace Emico\RobinHqLib\Client;
+namespace Emico\RobinHqLib\Client\Exception;
 
 use Exception;
 
 class InvalidApiResponseException extends Exception
 {
 
-}
+}