chore(node-version): upgrade minimal node version to 6

BREAKING CHANGE: Will no more work with node v4

.travis.yml

@@ -6,7 +6,7 @@ cache:
 language: node_js
 node_js:
   - "8"
-  - "4.7"
+  - "6"
 script: npm run test:ci
 after_success:
   - curl -Lo travis_after_all.py https://raw.githubusercontent.com/contentful/travis_after_all/master/travis_after_all.py

README.md

@@ -30,7 +30,7 @@ Browsers and Node.js:
 - Edge
 - IE11 (with [legacy version](#legacy-browsers) of the library)
 - Safari
-- node.js (4.x, 6.x)
+- node.js (6.x, 8.x)
 
 Other browsers should also work, but at the moment we're only running automated tests on the browsers and Node.js versions specified above.
 

package.json

@@ -113,7 +113,7 @@
     "webpack-bundle-analyzer": "^2.11.0"
   },
   "engines": {
-    "node": ">=4.7.2"
+    "node": ">=6"
   },
   "config": {
     "commitizen": {