Remove dependency on R >= 3.5

[gothub]

Building a vignette that uses caching (in a chunk, i.e. "{r, cache=TRUE}" causes knitr to create an .rds file for the chunk. The RDS format has changed recently, so these .rds files cause R check to declare that the package is now dependent on R >= 3.5.

Since this requirement is not desired, chunk caching will be discontinued in this package.

[gothub]

Fixed in commit 5f2c725