Update R dependency to 3.6

[jonathancallahan]

Note from Stuart:

I am giving a guest lecture for a UW Madison class this coming week and I wanted to demo your PWFSLSmoke package, as the students are supposedly using R quite a bit. For the installation, I wanted to confirm that per the docs R (>= 3.3) still works?
I am asking because I have a Dockerfile that has an R 3.5 image, and the PWFSLSmoke package has been failing due to the dependency on the R package latticeExtra which requires R >= 3.6 now. I can always fix my Dockerfile by using an r-base image instead of a debian one, however if I'm going to tell a class to install the package, I want to confirm they all need to be on R 3.6.