chore(package): update clean-css to version 3.4.20
chore(package): update clean-css to version 3.4.20

https://greenkeeper.io/
camptocamp/ngeo,kalbermattenm/ngeo,Jenselme/ngeo,ger-benjamin/ngeo,ger-benjamin/ngeo,adube/ngeo,camptocamp/ngeo,camptocamp/ngeo,adube/ngeo,Jenselme/ngeo,adube/ngeo,adube/ngeo,camptocamp/ngeo,kalbermattenm/ngeo,ger-benjamin/ngeo,Jenselme/ngeo,Jenselme/ngeo,kalbermattenm/ngeo,camptocamp/ngeo