Avoid "CRAN without setting a mirror"

Copy http://stackoverflow.com/a/8475208/1802726 as instruction to solve

```
$ make serve
Loading required package: knitr
Loading required package: stringr
Loading required package: checkpoint

checkpoint: Part of the Reproducible R Toolkit from Microsoft
https://mran.microsoft.com/documents/rro/reproducibility/
Installing missing required packages: ggplot2
Installing package into ‘/home/raniere/R/x86_64-pc-linux-gnu-library/3.3’
(as ‘lib’ is unspecified)
Error in contrib.url(repos, type) :
trying to use CRAN without setting a mirror
Calls: source ... generate_md_episodes -> install.packages -> grep -> contrib.url
Execution halted
make: *** [Makefile:84: lesson-rmd] Error 1
```