Luxembourg release

First release with next major libsass version
This may cause your existing sass code to break
Please check the libsass changelog for breaking changes
https://github.com/sass/libsass/releases/tag/3.6.0

• Bumped libsass version to latest 3.6 stable branch (3.6.0)
• Added digest plugin to compute md5/base64/crc checksums
• Adjusted a few test cases to match new 3.6 behaviors