avoid loading spdy if not used and prevent deprecation warning

Author: massimocandela

lib/server.js

@@ -11,7 +11,6 @@ var _ = require('lodash');
 var assert = require('assert-plus');
 var errors = require('restify-errors');
 var mime = require('mime');
-var spdy = require('spdy');
 var vasync = require('vasync');
 
 var Chain = require('./chain');
@@ -187,6 +186,7 @@ function Server(options) {
     ];
 
     if (options.spdy) {
+        var spdy = require('spdy');
         this.spdy = true;
         this.server = spdy.createServer(options.spdy);
     } else if (options.http2) {