adds `_gethostbyname` in `downloader.py` to fix #751
adds `_gethostbyname` in `downloader.py` to fix #751

`_gethostbyname` tries to resolve the public IP adress(es) using google's
public JSON API, patching `_config_dns` to recieve IP via `_gethostbyname`
in case of a `socket.gaierror` exception. See issue #751 for more details.

gboeing/osmnx,gboeing/osmnx