Set min node.js version to 6.0.0

In #861 support for
nodejs < 6 was dropped. However, the minimal version was to 6.11.5 for
no clear reason. This excludes version v6.11.4 from current Ubuntu 17.10
as well as version 6.10.3 running on AWS Lambda.

package.json

@@ -67,7 +67,7 @@
   "homepage": "https://github.com/jantimon/html-webpack-plugin",
   "repository": "https://github.com/jantimon/html-webpack-plugin.git",
   "engines": {
-    "node": ">=6.11.5"
+    "node": ">=6"
   },
   "config": {
     "commitizen": {