Postcss6 upgrade

.travis.yml

@@ -2,10 +2,9 @@ sudo: false
 
 language: node_js
 node_js:
-  - '6'
-  - '5'
-  - '4'
-  - '0.12'
+  - 8
+  - 6
+  - 4
 
 script:
   - npm run test

appveyor.yml

@@ -2,10 +2,9 @@
 
 environment:
   matrix:
+    - nodejs_version: 8
     - nodejs_version: 6
-    - nodejs_version: 5
     - nodejs_version: 4
-    - nodejs_version: 0.12
 
 version: "{build}"
 build: off

docs/content/usage.md

@@ -84,6 +84,13 @@ and show provide duplicates in their examples.
 (eg: autoprefixer + cssnext - but cssnext already includes autoprefixer).**  
 _In order to fix this, here is a warning. You are welcome._
 
+## `warnForDeprecations`
+
+_(default: true)_
+
+This option should be left with its default value, unless you are aware of the
+risk and plan to handle the situation.  
+
 ---
 
 **To know all available options, please check corresponding postcss plugin by