feat: Drop Node.js v7 configuration file

BREAKING CHANGE:

Author: robertrossmann

environments/nodejs/v7.js

@@ -9,11 +9,11 @@
 'use strict'
 
 /**
- * This file configures ESLint to work with the Node.js 7.0 release
+ * This file configures ESLint to work with the Node.js 8.0 release
  */
 module.exports = {
 
-  extends: './v7.js',
+  extends: './v6.js',
 
   parserOptions: {
     ecmaVersion: 2017,