...rebuild base images

There is currently one docker image pre-build on Dockerhub that has some requirements already installed which would otherwise take a lot of time during build, e.g. in TravisCI.

If you add or remove requirements please consider updating the docker base images to speed up the build process.

update R executor base image chris89/mpci_r

cd src/executionenvironments/r
docker build -f Dockerfile.base -t chris89/mpci_r .