Log Message:
libsass: updated to 3.6.1

3.6.1:
travis: add gcc-8 to CI
[build] enable concurrent compilation
Remove the link to ast_factory.hpp from Visual C++ project files
[build] add support of Visual Studio 2017 and 2019 to the Visual C++ project file
Makefile: Fix STATIC_LIBSTDCPP support
Remove abspath from native Makefile
Read files using for better portability
Add comparison operators for SharedImpl, fix bugs
VS2013 noexcept workaround