vimPlugins: backoff on timeout in update.py

Updating vim-plugins recently started timing out regularly for me. It
may have to do with an ISP switch on my side, but I don't think that
should cause timeouts. I guess I'm probably not the only one
experiencing this, so in this comment I introduce exponential backoff.
Every request will be retried up to 3 times (3 seconds, 6 seconds, 12
seconds delay).