To support the improved Github Pages, set your records up like this:
HOST NAME IP ADDRESS RECORD TYPE TTL
@ 192.30.252.153 A (Address) 1800
www <username>.github.io CNAME (Alias) 1800
Then, fill out one row under SUB-DOMAIN SETTINGS:
HOST NAME IP ADDRESS RECORD TYPE TTL
@ 192.30.252.154 A (Address) 1800
More information here: http://davidensinger.com/2013/03/setting-the-dns-for-github-pages-on-namecheap/