Update favicon

This commit replaces the default Octopress favicon with the one we are
using on the Causes production site.

Change-Id: Ibdb09fe8e8581718acb742b80367e6619706762c
Reviewed-on: https://gerrit.causes.com/32402
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>