Use PostCSS 7 & drop support for Node.js 4 (#372)

* Require Node.js 6

* Use PostCSS 7

Closes #369

Author: realityking

.travis.yml

@@ -1,5 +1,8 @@
 language: node_js
 node_js:
   - stable
-  - 4.5
+  - 10
+  - 8
+  - 6
+
 script: npm run ci

package.json

@@ -17,8 +17,11 @@
     "index.js",
     "lib"
   ],
+  "engines": {
+    "node": ">=6.0.0"
+  },
   "dependencies": {
-    "postcss": "^6.0.1",
+    "postcss": "^7.0.1",
     "postcss-value-parser": "^3.2.3",
     "read-cache": "^1.0.0",
     "resolve": "^1.1.7"