It fails at get-deps: https://gist.github.com/c2306b881cddc7fd2b95
I have it reproducing consistently on my computer and another agner contributor's computer as well. Not sure what's happening exactly — didn't have much time to look into this; but thought you might be interested to know.
It works the same way if agner is not used: simply git cloning the repo and running ./rebar get-deps produce the same error.
Somebody at #erlang said that your repo has an empty deps/sync for some reason, thus rebar doesn't bother pulling it.
I also bumped into this, I managed to get around it somehow. I think by manually cloning the sync repo.
It might have something to do with capitalization in the name of the Sync application.
There's something broken about the nitrogen repo as far as I can see — just look at this https://github.com/nitrogen/nitrogen/tree/master/deps
This is now fixed. I had accidentally checked in an empty directory under deps/sync which confused rebar's dependency checking.