Support for clean-css (https://github.com/GoalSmashers/clean-css/)
Fix AttributeError in offline compression
Fix reading UTF-8 files which have BOM

https://github.com/django-compressor/django-compressor/issues/567
Merge pull request #568 from JereMalinen/develop

Fix reading UTF-8 files which have BOM
Merge pull request #551 from vovanbo/feature/clean_css_support

Support for clean-css
Merge pull request #554 from cbjadwani/Fix_AttributeError_in_offline_compression

Fix AttributeError in offline compression
Merge remote-tracking branch 'upstream/develop' into develop
Replace COMPRESSOR_CACHE_BACKEND by COMPRESSOR_CACHE_ALIAS