Issue with PHP 5.6

Hello, I have an error when I'm trying to do a request (with PHP 5.6.40 and cybersource/rest-client-php 0.0.29)

```
Parse error: syntax error, unexpected '->' (T_OBJECT_OPERATOR) in /usr/src/myapp/vendor/cybersource/rest-client-php/lib/ApiClient.php on line 338
```