update importer to use soil built-in get_filename()
update importer to use soil built-in get_filename()

- dependent on https://github.com/dimagi/django-soil/pull/3/

puttarajubr/commcare-hq,SEL-Columbia/commcare-hq,puttarajubr/commcare-hq,dimagi/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,SEL-Columbia/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,gmimano/commcaretest,qedsoftware/commcare-hq,dimagi/commcare-hq,SEL-Columbia/commcare-hq,gmimano/commcaretest,gmimano/commcaretest