remove explicit openblas in R installation, replaced it by the shared Blas system, as used by the debian R package. it is now possible to switch Blas implementation using LD_LIBRARY_PATH or update-alternatives by example. Also added the --with-tcltk flag, that was already automatically setup to make it more explicit.

will see if this results in cache files that inflate image size signifcantly...

fix env var definition

apply latest fixes to all hooks

hopefully this will avoid annoyingly frequent build-errors on nightly builds of the versioned stack
 / ht https://unix.stackexchange.com/questions/375575/reliable-cdn-mirror-for-debian/375579#375579

Add libreadline7 to devel and 3.4.0

(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