mod validation is overly strict

When uploading a mod via the python-api with `url = ""` in the `mod_info.lua`, this exception is thrown:

```
File "/code/api/mods.py", line 85, in mods_upload
process_uploaded_mod(temp_mod_path)
File "/code/api/mods.py", line 206, in process_uploaded_mod
mod_info = parse_mod_info(temp_mod_path)
File "/usr/local/lib/python3.5/site-packages/faf/tools/fa/mods.py", line 52, in parse_mod_info
raise ValueError(errors)
ValueError: {'url': ['Not a valid URL.']}
```

That is probably too strict.