Fixed WooCommerce 2.0.18 Compatibility Issue

Removed line testing existance of 'woocommerce_payment_gateway', I
think it's safe to assume if WooCommerce is installed, the payment
gateways will be installed as well.

Moved WC version dection code inside the constructor. It was
referencing an non-existant $this.
Merge pull request #7 from ohryan/master

Fixed WooCommerce 2.0.18 Compatibility Issue