Dependency on R 3.6

[giuseppec]

I feel that the dependency on R v3.6 is too strict. I have seen many companies still having only R 3.5 (or earlier) installed so that I had to remove exercises that are easily solvable with DataExplorer in some courses. @boxuancui can you maybe describe for which functions R v3.6 is required? Maybe the backports package helps here to go back to a dependency on R v3.5 (or even earlier).