Add Causes.com to list of sites using Susy
Remove trailing slashes from site URLs

Some of the URLs in the list of sites using Susy had trailing slashes
while others did not. I sanded off the bonus trailing slashes for
consistency and simplicity. Save them bytes.

I left the trailing slash on the Concordia discors link, since removing
it would have resulted in a redirect every time it was followed.
Merge pull request #217 from lencioni/add-causes

Add Causes.com to list of sites that use Susy