Warn when pg.Pool() isn’t called as a constructor

in preparation for #1541. `eval` is a bit awkward, but it’s more accurate than an `instanceof` check and will work on platforms new enough to support pg 8 (i.e. only not Node 4).

lib/index.js

@@ -13,8 +13,19 @@ var defaults = require('./defaults')
 var Connection = require('./connection')
 var Pool = require('pg-pool')
 
+let hasNewTarget = false
+
+try {
+  eval('(function () { new.target })')
+  hasNewTarget = true
+} catch (error) {}
+
 const poolFactory = (Client) => {
   var BoundPool = function (options) {
+    if (hasNewTarget && eval('new.target') === undefined) {
+      process.emitWarning('Constructing a pg.Pool without new is deprecated and will stop working in pg 8.', 'DeprecationWarning', 'PG-POOL-NEW')
+    }
+
     var config = Object.assign({ Client: Client }, options)
     return new Pool(config)
   }