Fix access to $config variable in CasManager

Fix error introduced by commit a5c3785ed8d6a35868bc169f07e40e889087fd2e and reported on a comment in issue #93

src/Subfission/Cas/CasManager.php

@@ -77,7 +77,7 @@ protected function enableDebugCas() {
 			phpCAS::setLogger();
 		}
 		phpCAS::log( 'Loaded configuration:' . PHP_EOL
-			             . serialize( $config ) );
+			             . serialize( $this->config ) );
 	}
 	/**
 	 * Configure CAS Client|Proxy