to_file(): 'long' isn't a valid Fiona property type

[sgillies]

The question http://gis.stackexchange.com/questions/89206/geopandas-error-when-writing-to-file-valueerror-long-is-not-in-list revealed a bug to me.

If you pass schema={'geometry': 'Point', 'properties': {'foo': 'long'}} into fiona.open(), the type 'long' isn't found at https://github.com/Toblerity/Fiona/blob/master/src/fiona/ogrext.pyx#L973. OGR doesn't distinguish between long and int, so converting 'long' to 'int' within Fiona may help...

But :)

Fiona will always return 'int' in the .schema attribute and this could cause trouble for programs that pass 'long' and expect it to stick. So, let's fix up geopandas so it always uses 'int' and never 'long'.