AmazonFlexPay usage broke, so I fixed it

At one point, it seems that AmazonFlexPay returned an object on which you could call `#url`. It doesn't do that anymore (https://github.com/kickstarter/amazon_flex_pay/commit/512d077b563c573122a43392e875752ca78b4e86). It returns the url as a String. So, method_not_found errors (calling `String#url`) were a-flyin'. I updated the call to `AmazonFlexPay.multi_use_pipeline`, which you can now `redirect_to` directly, and removed the `@pipeline` and `@pipeline.url` business altogether.