Use closure-util's Closure lib for the check-requires target
Make compiler happy by replacing 0e6 by 0
compilation_level is now just ADVANCED and SIMPLE
Remove some externs declarations

All these declarations are defined in recent versions of Closure Compiler.
Get Closure Library from closure-util for host-examples
Refresh the compiler error list

Based on https://github.com/openlayers/closure-util/blob/master/compiler-options.txt
Use serve.js as the test server
Update .travis.yml
Target integration-test renamed to ci
Remove ununsed serve-integration-test target
Add a buildcfg/readme.md
Remove the base.json Plovr config file
Remove the test.json Plovr config file
Farewell Plovr