makes more sense to fix the build date when the R version is also set.  Leads to removal of images older than R-3.1.1,
since MRAN does not exist during 3.1.0 (nor did docopt). This also makes sure littler and the install2.r script are
available at all levels.

3.3.2 now uses current day as BUILD_DATE (like latest)
while 3.3.1 pins to last day that version was current (2016-10-31)

verse/Dockerfile builds on latest, verse/Dockerfile:3.3.2 builds on fixed date from tidyverse:3.3.2