simplify build

Author: Rich-Harris

.travis.yml

@@ -1,8 +1,6 @@
 language: node_js
 node_js:
-  - 7
-  - 6
-  - 4
+  - stable
 
 script:
   - npm test

appveyor.yml

@@ -1,9 +1,7 @@
 # Test against the latest version of this Node.js version
 environment:
   matrix:
-    - nodejs_version: "4"
-    - nodejs_version: "6"
-    - nodejs_version: "7"
+    - nodejs_version: "stable"
 
 # Install scripts. (runs after repo cloning)
 install: