Using ccache to speed up R package checks on Travis CI

Introduction The problem The mlr use case The solution Code Introduction Continuous integration checking for R packages is usually done on Travis CI because the R community has established a community driven build framework for R. In case you are not aware, there are also other tools that try to simplify the CI tasks for R even more. This works great for simple checking of small R packages but once it comes to packages that have a lot of dependencies, developers sometimes run into troubles regarding build time.