Log Message:
libsass: updated to 3.6.3

3.6.3:
This release fixes issues introduced with 3.6.2 and the extend backport.
Also fixes an issue with memory lifespan of error_src on the C-API.
Additionally some edge case crashes have also been addressed.

Changelog
Fix compound extend warning
Fix extend being stuck in endless loop
Fix various edge-case segfault crashes
Extend error_src lifetime on c-api context
Fix memory leak in permutation function
Preserve indentation in nested mode