Package dependency riverplot is no longer hosted on CRAN

```
art@aziraphale:~/git/dynsbmR$ R CMD INSTALL .
* installing to library ‘/home/art/R/x86_64-pc-linux-gnu-library/4.3’
ERROR: dependencies ‘riverplot’, ‘RColorBrewer’ are not available for package ‘dynsbm’
* removing ‘/home/art/R/x86_64-pc-linux-gnu-library/4.3/dynsbm’
```

Can probably replace this with ggfree