ci(R): Switch to R 3.6
ci(R): Switch to R 3.6

There seems be a problem with 4.0: https://dev.azure.com/stencila/stencila/_build/results?buildId=6712&view=logs&j=49d7886f-34c2-5c07-5656-fc498a255505&t=4f7d095f-bb17-54e5-abf5-403a7bb1dc28&l=26

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila