Used get_woocommerce_currency() to retrieve the currency to pass to Stripe

With the introduction of the Currency Switcher, WooCommerce now supports orders in multiple currency. Therefore, taking the base currency using get_option() is no longer possible. Function get_woocommerce_currency() always returns the currency in which the order has been placed.
Merge pull request #3 from daigo75/master

Used get_woocommerce_currency() to retrieve the currency to pass to Stripe