fix broke tests for node 0.12

Author: 刘祺

package.json

@@ -4,7 +4,7 @@
   "description": "PostCSS gulp plugin",
   "main": "index.js",
   "scripts": {
-    "pretest": "eslint index.js",
+    "pretest": "node-version-gte-4 && eslint index.js || echo \"ESLint not supported\"",
     "test": "mocha test.js"
   },
   "repository": {
@@ -33,6 +33,7 @@
     "eslint": "^3.19.0",
     "gulp-sourcemaps": "^2.6.0",
     "mocha": "^3.2.0",
+    "node-version-check": "^2.1.1",
     "proxyquire": "^1.7.11",
     "sinon": "^2.1.0"
   },