Fixing issues #63

Hi,
I thought this would fix issues #63: https://github.com/rgeo/activerecord-postgis-adapter/issues/63
So it would be possible to call:

    self.lonlat = Pin.rgeo_factory_for_column(:latlon).point(self.longitude, self.latitude)

Instead of:

     self.lonlat = Pin.rgeo_factory_for_column(:latlon, {}).point(self.longitude, self.latitude)

Let me know.

Silvia