Bring minimum R version into line with upstream

Author: jonclayden

DESCRIPTION

@@ -29,7 +29,7 @@ Description: R and 'Eigen' integration using 'Rcpp'.
  version 2 or later, as is the rest of 'Rcpp'.
 License: GPL (>= 2) | file LICENSE
 LazyLoad: yes
-Depends: R (>= 3.5.0)
+Depends: R (>= 3.6.0)
 LinkingTo: Rcpp
 Imports: Rcpp (>= 0.11.0), stats, utils
 Suggests: Matrix, inline, tinytest, pkgKitten, microbenchmark