Remove dependency to R > 3.5

See the following check result:

NB: this package now depends on R (>= 3.5.0)
     WARNING: Added dependency on R >= 3.5.0 because serialized objects in
     serialize/load version 3 cannot be read in older versions of R.
     File(s) containing such objects:
       'airGRiwrm/inst/vignettes/ParamV03.RDS'