requires syntax throws hg error
If you try to specify a git repo with a required version, nimble will think its pulling a mercurial repository.

Example:

```
requires "https://github.com/danielecook/BitVector#b8cc21271c90cca96ed31f5d5383711dc96a8d3f >= 0.4.11"
```

Results in this error:

```
     Error: 'hg' not in PATH.
```

It would be good to improve this error message or alternatively allow for version checking with specific commits OR disallow it and alert the user.