Simple SASS mixin using the @content directive used as a short-hand for the standard bootstrap 3.x.x media-queries

responsify.scss

@mixin responsify($target) {

  @if $target == large {
    @media screen and (min-width: 1200px) {
      @content;
    }
  }

  @if $target == medium {
    @media screen and (min-width: 992px) and (max-width: 1200px) {
      @content;
    }
  }

  @if $target == small {
    @media screen and (min-width: 768px) and (max-width: 992px) {
      @content;
    }
  }

  @if $target == mobile {
    @media screen and (max-width: 768px) {
      @content;
    }
  }
}