Bug in exifread.utils.get_gps_coordinates function (very easy to fix)

The following line in exifread.utils is wrong:
```
gps_tags = [lng_ref_tag_name, lng_tag_name, lat_tag_name, lat_tag_name]
```
and should be e.g.,:
```
gps_tags = [lng_ref_tag_name, lng_tag_name, lat_ref_tag_name, lat_tag_name]
```
Link to the line: https://github.com/ianare/exif-py/blob/d60f18d3b5f701cd87523c577adacd91175f1365/exifread/utils.py#L64