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

https://greenkeeper.io/
gingerik/ol3,thhomas/ol3,ahocevar/ol3,planetlabs/ol3,geekdenz/ol3,stweil/ol3,stweil/openlayers,openlayers/openlayers,geekdenz/ol3,tschaub/ol3,gingerik/ol3,openlayers/openlayers,openlayers/openlayers,klokantech/ol3,oterral/ol3,tschaub/ol3,planetlabs/ol3,thhomas/ol3,thhomas/ol3,stweil/openlayers,fredj/ol3,adube/ol3,ahocevar/openlayers,bjornharrtell/ol3,mzur/ol3,oterral/ol3,ahocevar/ol3,stweil/openlayers,tschaub/ol3,klokantech/ol3,oterral/ol3,bjornharrtell/ol3,geekdenz/openlayers,stweil/ol3,planetlabs/ol3,bjornharrtell/ol3,klokantech/ol3,ahocevar/ol3,fredj/ol3,geekdenz/openlayers,ahocevar/ol3,mzur/ol3,klokantech/ol3,adube/ol3,gingerik/ol3,stweil/ol3,thhomas/ol3,ahocevar/openlayers,fredj/ol3,geekdenz/ol3,mzur/ol3,ahocevar/openlayers,geekdenz/ol3,fredj/ol3,geekdenz/openlayers,adube/ol3,tschaub/ol3,gingerik/ol3,planetlabs/ol3,mzur/ol3,stweil/ol3