Explicitly ask for build_conf's master branch

something's going on at github, it's returning errors for all non-explicit
branch checkouts

$ git clone https://github.com/CyanogenMod/hudson
Cloning into 'hudson'...
remote: Counting objects: 1409, done.
remote: Compressing objects: 100% (835/835), done.
remote: Total 1409 (delta 751), reused 1206 (delta 560)
Receiving objects: 100% (1409/1409), 232.30 KiB | 233 KiB/s, done.
Resolving deltas: 100% (751/751), done.
warning: remote HEAD refers to nonexistent ref, unable to checkout.