Update postcss to 6.0

Author: yuezk

README.md

@@ -22,10 +22,18 @@
 
 ## Install
 
+For `postcss 6.x`
+
 ```sh
 npm install --save-dev postcss-urlrev
 ```
 
+and for `postcss 5.x`
+
+```sh
+npm install --save-dev postcss-urlrev@1.x
+```
+
 ## Usage
 
 ```js

package.json

@@ -20,17 +20,17 @@
   },
   "homepage": "https://github.com/yuezk/postcss-urlrev",
   "dependencies": {
-    "postcss": "^5.0.0"
+    "postcss": "^6.0.0"
   },
   "devDependencies": {
-    "chai": "^3.0.0",
-    "coveralls": "^2.11.11",
-    "express": "^4.13.3",
-    "gulp": "^3.9.0",
+    "chai": "^3.5.0",
+    "coveralls": "^2.13.1",
+    "express": "^4.15.2",
+    "gulp": "^3.9.1",
     "gulp-eslint": "^0.14.0",
-    "gulp-mocha": "^2.1.2",
-    "istanbul": "^0.4.4",
-    "mocha": "^2.2.5"
+    "gulp-mocha": "^2.2.0",
+    "istanbul": "^0.4.5",
+    "mocha": "^2.5.3"
   },
   "scripts": {
     "test": "mocha",