RStudio has moved

[philclifford]

Now the website redirects here and all the download info has been "improved" we no longer find the URL.

Working on a solution, but also considering adding rstudio-server ( #641 ) and maybe even r-base

NB testing hampered because local overrides were broken by 4dc0c33 ( see #639 ) , but
git checkout d274c36 ; ./deb-get show rstudio suggests a workable fix will be along shortly.