Chapter 15 failing

With this error on my desktop:

```
Quitting from lines 565-566 (15-eco.Rmd) 
Error: The backend of Task 'mongon' has been removed. Set `store_backends` to `TRUE` during model fitting to conserve it.

Execution halted
Error in Rscript_render(f, render_args, render_meta, add1, add2) : 
  Failed to compile 15-eco.Rmd
```

Heads-up @jannes-m 