Error during listing nimble package versions
Specs:
```
OS: Debian testing 64-bit, fully updated
```
```
Nim Compiler Version 0.19.0 [Linux: amd64]
Compiled at 2018-09-28
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: d993b9420bcf49f8e49acbb8169ff24f46bfc8df
active boot switches: -d:release
```

I run `nimble list --ver` and while was fetching the entire list, it threw the following error message:

![image](https://user-images.githubusercontent.com/1777466/46181898-10c37b00-c2d1-11e8-8859-fd0b1544045d.png)
