chore: Update .travis.yml

Remove Node.js 4, because it is not supported by webpack 4. Breaking change!

.travis.yml

@@ -3,14 +3,8 @@ os:
 language: node_js
 node_js:
   - "node"
-  - "lts/*"
-  - "4"
-
-before_script:
-  # Fixes an annoying bug in ESLint on Node 4
-  # https://github.com/babel/babel-eslint/issues/558
-  # TODO: Remove when Node 4 is not supported anymore
-  - npm install estraverse
+  - "8"
+  - "6"
 
 script:
   - npm test