Log Message:
Update ruby-sass to 3.4.20.

3.4.20 (09 December 2015)

* Fix a bug with the rounding changes from 3.4.14 and 3.4.15 where some
  negative numbers would incorrectly be rounded up instead of down.
* Better compression for :nth pseudoselectors with subtraction. Issue #1650
* Add support for the new supports() clause for CSS @import directives.
* Rounding numbers now respects Sass’s precision setting for numbers very
  close to half an integer.
* Add support for the q unit, representing one quarter of a millimeter.
* Mitigate a race condition when multiple threads are using the same
  Sass::Plugin object at once.
* In compressed mode, numbers between -1 and 1 now have the leading 0 omitted.
* Source maps now include source ranges for comments.

Deprecation – Must Read!

Certain ways of using #{} without quotes in property and variable values have
been deprecated in order to simplify the feature.

Currently, #{} behaves unpredictably. If it’s used near operators, it will
cause those operators to become part of an unquoted string instead of having
their normal meaning. This isn’t an especially useful feature, and it makes
it hard to reason about some code that includes #{}, so we’re getting rid of
it.

In the new world, #{} just returns an unquoted string that acts like any other
unquoted string. For example, foo + #{$var} will now do the same thing as foo
+ $var, instead of doing the same thing as unquote("foo + #{$var}").

In order to ease the transition, Sass will now emit deprecation warnings for
all uses of #{} that will change behavior in 4.0. We don’t anticipate many
warnings to appear in practice, and you can fix most of them automatically by
running sass-convert --in-place on the affected files.

For more details, see the blog post on the deprecation and the GitHub issue in
which it was planned.