fixes https://github.com/rocker-org/rocker/issues/299, thanks @dmenne 

ggplot2 3.0 suggests sf, we do not want tidyverse to become rocker/geospatial quite yet

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