extend tests for download_boundaries()

change default download_boundaries(..., method = "sf")

merge in changes to the master

update title again

Updating the travis for now to not test oldrel because of weirdness with sodium. Explanation from Rich: "having serialised an object with a more recent version of R than you are unserialising with So if you create the secret with R version 3.5.x it and re-add it it, things will work. Or wait a few weeks until oldrel becomes 3.6"