Properly parse negative numbers preceded by a comment.

This follows the CSS Syntax Level 3 spec for how unary minus is parsed.

See #1020
Do not freeze `true` or `false` in `Script::Value::Base.new`.

This fixes #1210, wherein the freezing of `true` causes strange side
effects in other gems. It also separates the `value.freeze` call from
the assignment `@value = value`, to emphasize that `value.freeze` is a
side effect that mutates the value, not a function that returns a frozen
copy of the value.
Avoid unnecessary interpolation in `sass-convert`.

Closes #1208
More efficient checks for freezability.
Add a changelog entry.
Merge branch 'dont-freeze-true-false' into stable

Conflicts:
	doc-src/SASS_CHANGELOG.md

Closes #1210
Bundle the canonical version of listen rather than my fork.

The canonical version has merged in the patch in my fork that we rely
on.
Merge branch 'stable' of github.com:nex3/sass into stable