Dockerfile: COPY -> ADD

```
2015/01/07 21:57:16 COPY has been deprecated. Please use ADD instead
```

Thanks @inconshreveable

Dockerfile

 FROM golang:1.3
 MAINTAINER Brian Tiger Chow <btc@perfmode.com>
 
-COPY . /go/src/github.com/jbenet/go-ipfs
+ADD . /go/src/github.com/jbenet/go-ipfs
 RUN cd /go/src/github.com/jbenet/go-ipfs/cmd/ipfs && go install
 
 EXPOSE 4001 5001 4002/udp