Update lib version dependencies (for DiagrammeR mainly)

Fix @export tag in each R file (for Roxygen 5, otherwise it doesn't work anymore)
Regerate Roxygen doc

R-package/R/xgb.save.R

@@ -16,7 +16,6 @@
 #' bst <- xgb.load('xgb.model')
 #' pred <- predict(bst, test$data)
 #' @export
-#' 
 xgb.save <- function(model, fname) {
   if (typeof(fname) != "character") {
     stop("xgb.save: fname must be character")