[R-package] package documentation using roxygen2 7.0.0+

`roxygen2` 7.0.0 was [recently released](https://twitter.com/hadleywickham/status/1194338595202576385?s=19) to CRAN. To close this issue, install `roxygen2` locally and then change the R package's code so that the following works:

```
export CXX=/usr/local/bin/g++-8
export CC=/usr/local/bin/gcc-8
Rscript build_r.R
cd R-package
Rscript -e "roxygen2::roxygenize(load = 'installed')"
```