3rd attempt to replace dependency on rgdal with sf (issue #38)

[iantaylor-NOAA]

Commit 348d9a8 adds sudo: required to .travis.yml in the hopes that this is the final piece of the puzzle to close issue #38. Based on https://docs.travis-ci.com/user/reference/overview/ this may slow down the automated testing a little bit, but that probably doesn't matter.

[iantaylor-NOAA]

I've learned two valuable things:

• that I can flip a switch at https://travis-ci.org/profile/ to have Travis-CI test my fork of the package, so I don't have to submit a pull request to do that testing.
• that a commit to my fork was automatically included in this pull request so I didn't need to be deleting the pull requests and replacing them, but anyone watching the repository will get a notification with each push

Therfore, rather than continue causing a bunch of you to get notification every time I try something else, I'm going to close this pull request and deal with testing in my own fork.