Libsass Security vulnerability for 3.5.5

[sdmiller7]

Seeing multiple security vulnerabilities reported for libsass 3.5.5. Suggested bump to 3.6.0.

Reference Whitesource, redhat etc.
https://www.whitesourcesoftware.com/vulnerability-database/CVE-2018-19827

High Severity
In LibSass 3.5.5, a use-after-free vulnerability exists in the SharedPtr class in SharedPtr.cpp (or SharedPtr.hpp) that may cause a denial of service (application crash) or possibly have unspecified other impact.

[nschonni]

#2685