Merge pull request #81 from rocker-org/sandbox

remove stale cran mirror lists during cleanup (#80)
Merge pull request #85 from rocker-org/sandbox

add file to set of packages installed with RStudio (closes #84)
Merge pull request #87 from rocker-org/sandbox

switch to jessie and drop dist-upgrade
move a few things around for improved cache

- separated the non-essential bits from the essential bits so that the former can be cached completely
- added an explicit pinned version for the packages from `src:r-base` so there's a natural cache-bust when it's time to update
- moved several blocks higher in the file so they can be cached completely as well
- added a default command, so that if someone does `docker run -it --rm r-base`, they're dropped straight into a useful R shell
Merge pull request #88 from infosiftr/cacheability

move a few things around for improved cache
add CRAN Debian repo
Merge pull request #89 from rocker-org/feature/cran-apt-repo

add CRAN Debian repo
adding --force-yes as we haven't added the GPG key of CRAN yet
register key for apt repo on CRAN mirrors
add/test travis
travis badge
Merge pull request #91 from rocker-org/feature/travis-ci

Feature/travis ci
add option to enable root for user, #94
Merge
build r-devel and rstudio from official r-base
run base package testing on Travis-CI as in #8