Add minimum package requirement

[edgararuiz]

Older versions of dplyr, or dbplyr, or both could return the following error when attempting to collect data from a table via the Spark connection:

Error in `[<-`:
! Assigned data `map(.subset(x, unname), vectbl_set_names, NULL)` must
  be a vector.
Caused by error in `vec_size()`:
! `x` must be a vector, not a <numpy.ndarray/python.builtin.object> object.