fix(checkbox): Reduce $md-checkbox-transition-duration to 90ms

This commit reduces the amount of time checkbox transitions take, at the
recommendation of the designers from the material design team.

Closes #226.

Closes #230

src/components/checkbox/checkbox.scss

@@ -15,7 +15,7 @@ $md-checkbox-mark-color: md-color($md-background, background);
 /** The color that is used as the checkbox background when it is checked. */
 $md-checkbox-background-color: md-color($md-primary, 500);
 /** The base duration used for the majority of transitions for the checkbox. */
-$md-checkbox-transition-duration: 140ms;
+$md-checkbox-transition-duration: 90ms;
 /** The amount of spacing between the checkbox and its label. */
 $md-checkbox-item-spacing: 4px;