Bump LibSass to 3.5.5

git-subtree-dir: src/libsass
git-subtree-split: 39e30874b9a5dd6a802c20e8b0470ba44eeba929

Fixes #2538

The package `extend 3.0.1`, which is a dependency of `request 2.87` has a vulnerability :
https://hackerone.com/reports/381185

Upgrade `request` to v.2.88 will install `extend` v.3.0.2, the fixed version.

Fix #2496

Closes #2355

This forces a local compilation (slow) when they just want to
download right binary (fast).

Request has been updated so this irrelevant

As of 2.87.0 the hoek packages has been removed which restores
Node < 4 support

Copied from main ruby-sass