Log Message:
Update ruby-sass to 3.4.18.

3.4.17 (21 August 2015)

* Allow passing calc values to rgb/hsl color constructors.
* The source map end character for lists now correctly uses the end of the
  list instead of the end of the first element in the list.
* Fix up some edge cases where extra semicolons could be added to unknown
  directives in compressed mode.
* If you try to do a stupid color operation with a stupid value, the error
  message will now be less stupid than it was.
* Make is-superselector("a > c d", "a > b c d") return \ 
false. This also fixes
  some related @extend behavior.
* A / in a parenthesized list is consistently treated as a plain / unless it
  meets the criteria for being treated as division.
* In sass-convert, ensure that literal /es are preserved.

3.4.16 (10 July 2015)

* When converting from Sass to SCSS or vice versa, double-space around nested
  rules the same as around top-level rules.
* Compatibility with listen 3.
* Parse a minus operator with no surrounding whitespace (e.g. 20px-10px) the
  same regardless of whether it’s being used in a static property.

3.4.15 (22 June 2015)

* Further improve rounding for various numeric operations.
* Be more explicit in the function documentation about functions being
  immutable.
* Improve rounding in operator-based color operations.

Deprecations – Must Read!

* When using --stdin with the sass or scss executables, importing relative to
  the working directory is deprecated. Having the working directory on the
  load path was deprecated in 3.3 and removed in 3.4, but due to an oversight
  the deprecation process never happened for files read over standard input in
  particular. This is the first step of that process.