Fix test related to postcss-color-rgba-fallback update

Author: MoOx

src/__tests__/fixtures/features/color-rgba.expected.css

@@ -1,5 +1,5 @@
 .foo {
-  background: #99DD99;
+  background: #99dd99;
   background: rgba(153, 221, 153, 0.8);
   border: solid 1px #646667;
   border: solid 1px rgba(100,102,103,.3);