Update gateway options URL

Update the gateway options url to use `GET /v1/gateways_options.xml`
instead of `OPTIONS /v1/gateways.xml`.

Bump version

Version 2.0.24

Version 2.0.23

Version 2.0.22

Bump version to 2.0.21

Release v2.0.20

Release v.2.0.19

Bump version to 2.0.18

Add support for initiating offsite purchases

Closes #72