update_check broken, `lastdownstart` and `lastdownend` added in the latest API responses

[marinpurgar]

The lastdownstart and lastdownend properties were added to the latest API responses breaking the update_check method.

The error while trying to update the pingdom check:

pypingdom.api.ApiError: pingdom.ApiError: HTTP `400 - Bad Request` returned with message, "Invalid parameter: lastdownstart."

There is a fix for this issue in the #116 PR.

[lawrencekoenig]

The APP hasn't been updated since last year. Is there a plan to update the app for usage?