Serve current commit price with no caching

As complained about here:
http://www.reddit.com/r/Bitcoin/comments/1yu6l7/someone_just_sent_5000_in_btc_to_open/cfnsp03

The need to explicitly forbid caching is explained here:
https://github.com/github/markup/issues/224

FREEBIE
Merge pull request #33 from backspace/break-current-commit-price-cache

Serve current commit price with no caching
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. My theory is that
it is not re-reading the headers correctly, although it may be a bug
with github's fastly configuration, since they probably don't expect
anyone to be serving dynamic content in their READMEs.

This commit simply adds a trailing / to the image path, which
effectively forces fastly to re-fetch the image. Hopefully this time
it will read (and respect) the Cache-Control header.