Change default listen parameter to undefined

This will ensure our default listen address is whatever the Node.js
default is. It's currently set to listen on all addresses, and probably
won't change.

index.js

@@ -1,7 +1,7 @@
 
 const app = require('./app'),
     port = ( process.env.PORT || 3100 ),
-    host = ( process.env.HOST || 'localhost' );
+    host = ( process.env.HOST || undefined );
 
 /** run server on the default setup (single core) **/
 console.log( `pelias is now running on ${host}:${port}` );