perf: drop support old node

Author: Scrum

.travis.yml

@@ -6,8 +6,6 @@ node_js:
   - "8"
   - "7"
   - "6"
-  - "5"
-  - "4"
 
 after_script:
   - './node_modules/.bin/nyc report --reporter=text-lcov | ./node_modules/.bin/coveralls'

package.json

@@ -13,7 +13,7 @@
   },
   "main": "lib/index.js",
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "scripts": {
     "version": "conventional-changelog -i changelog.md -s -r 0 && git commit -am \"build: update changelog\"",

readme.md

@@ -16,7 +16,7 @@ Adds the ability to use a custom property in the options, at-rules.
 $ npm install postcss-at-rules-variables
 ```
 
-> **Note:** This project is compatible with node v4+
+> **Note:** This project is compatible with node v6+
 
 ## Usage