All you need is here :
We need to lower pin preference
sudo nano /etc/apt/preferences
Add these lines
Package: *
Pin: release n=jessie
Pin-Priority: 998
Save with Ctrl+X, Y and Enter
Now update packages and install
sudo apt-get update
sudo apt-get install libmono-cil-dev -y
After this, everything is ok. You can remove or empty the preference file. (and your sonarr database is still alive !!!)
For the future update, remove sonarr repository from /etc/apt/sources.list. 