fix networks var in discoverPeers() to match what might be imported

lib/PeerManager.js

@@ -223,8 +223,8 @@ PeerManager.prototype.discoverPeers = function(callback) {
   var self     = this;
   var async    = imports.async || require('async');
   var dns      = imports.dns || require('dns');
-  var networks = imports.networks || require('../networks')[this.config.network];
-  var seeds    = networks.dnsSeeds;
+  var networks = imports.networks || require('../networks');
+  var seeds    = networks[self.config.network].dnsSeeds;
 
   var dnsExecutor = seeds.map(function(seed) {
     return function(done) {