Remove dependency on purrr?

[andrie]

The package purrr has nested dependencies on R versions later than R-3.1.0.

To backport support for R-3.1.0 we may have to remove the dependency on purrr.

For example:

> if(!require("dplyr")) install.packages("dplyr")
Loading required package: dplyr
Failed with error:  ‘This is R 3.1.0, package ‘dplyr’ needs >= 3.1.2’
Installing package into ‘C:/Users/adevries/Documents/R’
(as ‘lib’ is unspecified)
Warning in install.packages :
  package ‘dplyr’ is not available (for R version 3.1.0)