ip.py: 'region_name' should be 'region_code'

This commit fix this issue
```
12:39  _Tux_> .ip 8.8.8.8
12:39  SuperLuserv2> Downloading GeoIP database, please wait...
12:39  SuperLuserv2> KeyError: 'region_name' (file "/usr/local/lib/python2.7/dist-packages/willie/modules/ip.py", line 99, in ip)
```