✨ recommend node v6 (#8674)

no issue

- see https://docs.ghost.org/docs/supported-node-versions
- add supported node versions link to README
- run coverage for node v6

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 # When changing node version also update it on lines 34, 36 and 46.
 node_js:
-  - "4"
   - "6"
+  - "4"
 
 sudo: false
 
@@ -27,7 +27,7 @@ env:
 
 matrix:
   include:
-    - node_js: "4"
+    - node_js: "6"
       env: TEST_SUITE=lint
   fast_finish: true
 
@@ -45,7 +45,7 @@ install:
 after_success:
   - |
       if [ "$TRAVIS_PULL_REQUEST" = "false" ]; then
-        if [[ "$DB" = "sqlite3" && "$TRAVIS_NODE_VERSION" = "4" ]]; then
+        if [[ "$DB" = "sqlite3" && "$TRAVIS_NODE_VERSION" = "6" ]]; then
           echo "Generate coverage..."
           grunt coverage
           npm install -g codeclimate-test-reporter

README.md

@@ -6,6 +6,7 @@
 The project is maintained by a non-profit organisation called the **Ghost Foundation**, along with an amazing group of independent [contributors](https://github.com/TryGhost/Ghost/contributors). We're trying to make publishing software that changes the shape of online journalism.
 
 - [Ghost.org](https://ghost.org)
+- [Supported Node Versions](https://docs.ghost.org/v1.0.0/docs/supported-node-versions)
 - [Latest Release](https://ghost.org/developers/)
 - [Help & Support](http://help.ghost.org/)
 - [Theme Docs](http://themes.ghost.org/v1.0.0/)