Add test for r3.6
ADD
tests/r/r3.6/install.R
+install.packages("ggplot2")

ADD
tests/r/r3.6/runtime.txt
+r-3.6-2018-07-07

ADD
tests/r/r3.6/verify
+#!/usr/bin/env Rscript
+library('ggplot2')
+
+print(version)
+# Fail if version is not 3.6
+if (!(version$major == "3" && as.double(version$minor) >= 6 && as.double(version$minor) < 7)) {
+ quit("yes", 1)
+}

Add test for r3.6