Update in setup.cfg: license-file -> license_files

This happens both in version 0.5.0 and in master:
```
 * QA Notice: setuptools warnings detected:
 * 
 * 	The license_file parameter is deprecated, use license_files instead.
 * 	Usage of dash-separated 'license-file' will not be supported in future versions. Please use the underscore name 'license_file' instead
 ```