go 1.0.x support

Unfortunately, due to the dependency on `code.google.com/p/go.tools/go/vcs`, `godep` cannot be built w/ go versions prior to `1.1`.

```bash
$ go get github.com/kr/godep
# code.google.com/p/go.tools/go/vcs
../../../code.google.com/p/go.tools/go/vcs/discovery.go:20: function ends without a return statement
../../../code.google.com/p/go.tools/go/vcs/discovery.go:31: function ends without a return statement
```

I'm not sure if it was your intention to support only go 1.1+, but it would be nice if this was usable on the lowest common denominator.

It's unlikely that they'll accept a patch to resolve the above issues, and it seems like it is just used to detect what VCS each import uses, right?  Would you consider a patch which eliminates this external dependency so that `godep` can be used with older go installations?