replace httr::GET() with httr:RETRY()

[annnvv]

Thanks for this awesome project @walkerke!

In this PR, I'd like to propose swapping out calls to httr::GET() etc. with httr::RETRY(). This will make the package more resilient to transient problems like brief network outages or periods where the service(s) it hits are overwhelmed. In my experience, using retry logic almost always improves the user experience with HTTP clients.

I'm working on chircollab/chircollab20#1 as part of Chicago R Collab, an R 'unconference' in Chicago.

[walkerke]

Thank you for your PR! I've tested this out, and tigris already implements equivalent functionality using a while() loop with a custom message. This PR supercedes that with a less informative message so I'm not going to merge. Appreciate it regardless, however!