Update ruby-sass to 3.4.21.

3.4.21 (11 January 2016)

This is a bug fix release.

* Consistent output formatting for numbers close to an integer.
  Issue #1931

* Correctly round negative numbers that were almost but not quite a whole
  number (slightly greater than the negative number).
  Issue #1938

* Don't strip escaped semicolons from compressed output.
  Issue #1932

* Only compress around dashes within nth selectors.
  Issue #1933

* Selector compression of whitespace around commas was affecting attribute
  values.
  Issue #1947

* Make subtraction work when a unit is followed directly by a hyphen and then
  a period. For example, 1em-.75em now returns 0.25em rather than
  1em-0.75em. This is consistent with the behavior when the subtrahend begins
  with a 0.
  Issue #1954

(taca)