Exit read_crs early when there's no projection.
Exit read_crs early when there's no projection.

This wraps up #117 for good.

njwilson23/rasterio,snorfalorpagus/rasterio,clembou/rasterio,kapadia/rasterio,perrygeo/rasterio,clembou/rasterio,njwilson23/rasterio,brendan-ward/rasterio,youngpm/rasterio,brendan-ward/rasterio,johanvdw/rasterio,youngpm/rasterio,johanvdw/rasterio,kapadia/rasterio,njwilson23/rasterio,perrygeo/rasterio,clembou/rasterio,kapadia/rasterio,perrygeo/rasterio,johanvdw/rasterio,brendan-ward/rasterio,youngpm/rasterio,sgillies/rasterio