Get invoice to get updated transaction total for trial.

[rvdsteege]

Fix #13.

[remcotolsma]

Let's wait for a response from MemberPress:
#13 (comment)

[remcotolsma]

No fix from MemberPress unfortunately:

We have passed your feedback on to the development team. They have
logged it and will keep it in mind, but at this time have not road
mapped any changes.

As I mentioned in our previous conversation, there is a way to get the
information for the proper transaction amount through the trial amount
on the subscription object. I recommend updating your code since it
won't be changed in MemberPress for some time.

I will merge this pull request.