Fix lint error.

Author: mmalerba

src/lib/slider/slider.scss

@@ -17,7 +17,7 @@ $md-slider-thumb-arrow-gap: 12px !default;
 
 $md-slider-thumb-label-size: 28px !default;
 
-$md-slider-tick-color: rgba(0, 0, 0, .6) !default;
+$md-slider-tick-color: rgba(0, 0, 0, 0.6) !default;
 $md-slider-tick-size: 2px !default;