dub: Could not resolve configuration for package demo

[mw]

OK, looks something wrong with dub / or vibe-d 0.10.0

https://github.com/msgpack-rpc/msgpack-rpc-d/blob/master/examples/with_http_server/dub.sdl#L5

with
```
dependency "vibe-d" version="~>0.7.25"
```

`dub build` can at least starts.

But

```
dependency "vibe-d" version="~>0.10.0"
```


```
$ dub build
Error Could not resolve configuration for package with_http_server

```