Fix error in new automated release process
Fix error in new automated release process

If we don't check out master ourselves then when we try to
push to it later, we'll get:

"src refspec master does not match any"

sgebbie/ponyc,ponylang/ponyc,dipinhora/ponyc,sgebbie/ponyc,jemc/ponyc,dipinhora/ponyc,dipinhora/ponyc,ponylang/ponyc,sgebbie/ponyc,jemc/ponyc,sgebbie/ponyc,ponylang/ponyc,jemc/ponyc,sgebbie/ponyc