Trivially update commit payout image path FREEBIE

Bithub is now serving the payout image with Cache-Control: max-age=0 but github/fastly is still serving the old cached image on READMEs for Bithub, TextSecure, etc... Evidently fastly is not re-reading the headers correctly.

This commit simply adds a trailing / to the image path, which changes the url hash and effectively forces fastly to re-fetch the image and its headers. I've confirmed that the README in my fork is no longer caching the image.