Adds centOS dependencies to README

Running /bin/hubot on CentOS 6 failed for me initially because:
1. I had a "pre" release of node
2. I didn't have libicu-devel installed

This PR calls this out in the README.  This would have been useful to me, but I'm not crazy about how it clutters up the clean and terse setup instructions.  Maybe it should go somewhere else?  It was easy enough to troubleshoot and fix that maybe it doesn't even need to be called out.  Just thought I'd throw it up there in case it was useful.