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