Use GitHub contributors link for attribution

The commit log of a project often doesn't tell the whole story on contributors. The `contributors` page on GitHub will give a historical overview of all the activity in the repo.

* Link to the contributions page

Author: Harlow Ward

README.md

@@ -7,7 +7,7 @@ Other portions are copyright of their respective authors.
 
 ## Authors
 
-See https://github.com/rubycas/rubycas-server/commits
+See https://github.com/rubycas/rubycas-server/graphs/contributors
 
 ## Installation