Fix build task for concrete5 5.7

Fixes #689
Fix too much cleanup
Merge branch 'fix-grunt-build-task' of https://github.com/mlocati/concrete5-5.7.0 into mlocati-fix-grunt-build-task
tweak to composer
Merge branch 'mlocati-fix-grunt-build-task'
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0