[SPARKR][BUILD] AppVeyor change to latest R version

## What changes were proposed in this pull request?

R version update

## How was this patch tested?

AppVeyor

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #18856 from felixcheung/rappveyorver.

dev/appveyor-install-dependencies.ps1

@@ -114,7 +114,7 @@ $env:Path += ";$env:HADOOP_HOME\bin"
 Pop-Location
 
 # ========================== R
-$rVer = "3.3.1"
+$rVer = "3.4.1"
 $rToolsVer = "3.4.0"
 
 InstallR