runtime: enforce minimal node version to 6.9.0

Etherpad 1.6.6 does not run on node <= 5 already.
Node 6.9 is the first LTS release in the 6 series, and comes with npm 3.10.8.

Declarations in package.json are advisory unless the user has set
`engine-strict` config flag.

Updated the docs accordingly.

src/node/server.js

@@ -24,6 +24,7 @@
 var log4js = require('log4js')
   , async = require('async')
   , stats = require('./stats')
+  , NodeVersion = require('./utils/NodeVersion')
   ;
 
 log4js.replaceConsole();
@@ -39,6 +40,11 @@ var settings
 var npm = require("npm/lib/npm.js");
 
 async.waterfall([
+  function(callback)
+  {
+    NodeVersion.enforceMinNodeVersion('6.9.0', callback);
+  },
+
   // load npm
   function(callback) {
     npm.load({}, function(er) {