Fix dependency of ParseRepositoryTag

cause updating of dotcloud/docker, ParseRepositoryTag was moved.
So `RUN make` fails.

My environment is ubuntu 14.0.4.

```
root@a6db11b76295:/gocode/src/github.com/drone/drone# go build -o bin/drone github.com/drone/drone/cmd/drone
# github.com/drone/drone/pkg/build/docker
pkg/build/docker/image.go:66: undefined: "github.com/dotcloud/docker/utils".ParseRepositoryTag
```