Makes sure ranger method can generate grid of length 1. (topepo#1307)

* Update ranger.R

Currently, ranger gives an error if passed with `trainControl(method = "none")`. You can see the error here:

```
rm( list = ls())
n <- 1e3
x <- cbind(x = rnorm(n))
y <- c(x + rnorm(n))
train(x, y, method = "ranger", trControl = trainControl(method = "none"))
```

This was happening because the minimal grid was a length 2, as it was passing both `srule` and "extratrees" to expand.grid regardless of the argument `len`. 

The proposed change makes sure expand.grid will have length 1, if `len = 1`.

* reup model file

---------

Co-authored-by: Max Kuhn <[email protected]>

models/files/ranger.R

@@ -27,7 +27,7 @@ modelInfo <- list(label = "Random Forest",
                                                           classification = is.factor(y),
                                                           len = len),
                                          min.node.size = ifelse( is.factor(y), 1, 5),
-                                         splitrule = c(srule, "extratrees"))
+                                         splitrule = c(srule, "extratrees")[1:min(2,len)])
                     } else {
                       srules <- if (is.factor(y))
                         c("gini", "extratrees")