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

It is not clear from the `--help` docs that `--source-map` requires a boolean or path. Passing the flag alone results in a TypeError. There's an argument to be made that passing the flag alone should imply `true`, or at the very least it should fail with a helpful error, not a cryptic TypeError. For now, this PR fixes the --help docs so that they match the README.