Switch dependency of twitteR to rtweet

[DGaffney]

The maintainer of the twitteR package posted a deprecation notice 7 months ago on that repo's page (https://github.com/geoffjentry/twitteR), and the issues this library causes (https://github.com/geoffjentry/twitteR/issues/90) seem to be very hard to fix on a fresh install here in March 2017 - any way to move to a different package, such as https://github.com/mkearney/rtweet ?