Depends on closure-util version 0.18.0
Fix the closure-library version

The version is the same as the one provided by plovr-81ed862.jar
Merge pull request #2283 from fredj/closure-util.json

Fix the closure-compiler and closure-library versions
Export ol.source.Vector.clear()
Merge pull request #2285 from tsauerwein/vector-source-clear

Export ol.source.Vector.clear()