fix(third_party): don't intermix go-raft

the previous commit added both benbjohnson and coreos go-raft repos.
This is just asking for trouble. Only use the coreos version in the
third_party directory and maintain the coreos/master branch.
bump(github.com/coreos/go-raft): e129a0807cdcbd53440c1d722907043d06c16f1b
Merge pull request #58 from philips/update-third-party

fix(third_party): don't intermix go-raft