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`.

In order to submit a pull request, I had to set:

```bash
export HUB_TEST_HOST=myserver.mydomain:80
```

though probably if the rest of the code in #247 were merged in, then this wouldn't be necessary.

## Test results

This is just cucumber; I get errors about mocking stuff when I run `script/test`.

```
 ❯ cucumber
Using the default profile...
..............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

177 scenarios (177 passed)
1060 steps (1060 passed)
0m54.332s
>>> elapsed time 56s
```