Change handling of declined pledges if required to make sense

[hhyyrylainen]

The API documentation says:

declined_since indicates the date of the most recent payment if it failed, or `null` if the most recent payment succeeded. A pledge with a non-null declined_since should be treated as invalid. 

but I noticed a user with a really recent creation date but then a few hours later they were declined. So perhaps we should change the declined since check to check that the declined since time is in the future (or in the last 4 hours)?