Let's use shelljs instead of rimraf+execSync

- shelljs is a powerful and complete package
- execSync requires node-gyp that requires python... Quite too much...
Use shelljs instead of rimraf
build-release-clean fixes

- avoid creating a global variable "walk"
- don't continue execution if errors are raised
build-release-finish fixes

- use shelljs instead of rimraf+exec-sync
- fix handling case when version is not found
- echo some message so that users know what's happening
Merge branch 'fix-build-release-grunt-tasks' of https://github.com/mlocati/concrete5 into mlocati-fix-build-release-grunt-tasks