remove tidyverse dependence

[adw96]

An email from the amazing Hadley

Dear Amy D Willis,

Your package, rigr, lists the tidyverse in either Depends,
Imports, or Suggests in the DESCRIPTION file. This is a bad idea because,
in short, the tidyverse is a set of packages designed for interactive data
analysis, and it includes a very large number (>80) of direct and indirect
dependencies, most of which your package probably doesn't use.

Instead of depending on the entire tidyverse, please import from, suggest,
or depend on the packages in the tidyverse that you actually use. This
will make your package faster to install and will head off potential
problems down the road.

Thanks!

Hadley

Should be easy as (I think) we only need dplyr.

[adw96]

addressed in #170