chore(package): update clean-css to version 3.4.18
chore(package): update clean-css to version 3.4.18

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