update test to ensure postcss 3.0 compatibility

Author: MoOx

package.json

@@ -25,7 +25,7 @@
   "devDependencies": {
     "jscs": "^1.6.2",
     "jshint": "^2.5.6",
-    "postcss": "^2.2.5",
+    "postcss": "^3.0.0",
     "tape": "^3.0.0"
   },
   "scripts": {