Constructor deprication warning

Created by: jw35

Travis-CI builds for PHP 7.0 and 7.1 are reporting:

PHP Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; Ucam_Webauth has a deprecated constructor in /home/travis/build/cambridgeuniversity/ucam-webauth-php/ucam_webauth.php on line 30