fix env var definition

apply latest fixes to all hooks

(Debian 9.0, release 2017-06-17) for 3.4.0, latest, and devel

* test for building newer versions of r-ver on debian:stretch
* setup newer rstudio images for stretch
* lock older versions to jessie, fix PKI on stretch versions (requires dev version from r-forge)
* use dev version of `littler` `install2.r` script to avoid erroneous error on warning
* Mention stretch vs jessie use in README

remove comment about profvis as no longer installed, and add comment about remotes

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)