Support currency option on `#verify_on_gateway` method

[carloslopes]

This option is acceptable by the API but the lib doesn't allow us to send it.

[FairfullB]

+1 to this one, looks like there is an existing PR for this one open: https://github.com/spreedly/spreedly-gem/pull/104

@jknipp Any chance of that PR getting merged?

[jknipp]

@spreedly/developer-experience Can you address this request?

[bradbroge]

While we don't have a precise timeline right now, we should be able to review/approve and deploy updates before the end of September.