Use WC `get_option` method rather than WP global

The missing `$this->` for setting the `live_publishable_key` for the class causes this plugin to not work for live deployments as it fails to set the key resulting in using an empty string for the live publishable stripe key.

As the mistake was not there for the test keys this led to immense confusion as it worked fine on my test environment and subsequently failed when pushed live.