Update build versions, document in readme

Author: clayreimann

.travis.yml

@@ -1,9 +1,9 @@
 sudo: false
 language: node_js
 node_js:
-  - '6'
-  - '5'
-  - '4'
+  - '11'
+  - '10'
+  - '8'
 
 cache:
   directories:

README.md

@@ -86,10 +86,7 @@ npm install github-api
 ```
 
 ## Compatibility
-`Github.js` is tested on `Node.js`:
-* 6.x
-
-Note: `Github.js` uses Promise, hence it will not work in `Node.js` < 4 without polyfill.
+`Github.js` is tested on node's LTS and current versions.
 
 [codecov]: https://codecov.io/github/github-tools/github?branch=master
 [docs]: http://github-tools.github.io/github/