update test to ensure postcss 3.0 compatibility

Author: MoOx

package.json

@@ -19,7 +19,6 @@
   "files": [
     "CHANGELOG.md",
     "LICENSE",
-    "README.md",
     "index.js"
   ],
   "dependencies": {
@@ -29,7 +28,7 @@
   "devDependencies": {
     "jscs": "^1.6.2",
     "jshint": "^2.5.6",
-    "postcss": "^2.2.5",
+    "postcss": "^3.0.0",
     "tape": "^3.0.0"
   },
   "scripts": {