Allow browse to work with Enterprise server with only HTTP

This borrows code from the excellent work done by @mmrobins at https://github.com/github/hub/pull/247 -- It appears that some of the code changed so his patch may not apply cleanly anymore. Also his does a few more things which makes it longer and I thought that might be contributing to the delay in merging it perhaps. So I took what he had and made a very small patch just for this one feature (make `hub browse` work for an HTTP-only installation). My hope is that since this patch is small and simple, maybe it stands a better chance of getting merged and not sitting a long time like #247 did.

To do this, set:

```yaml
uri_scheme: http
```

in `~/.config/hub`.