test(ci): allow tests to run on Node 8.0.0->8.5.0

Early versions of Node.js 8 had a regression around the handling of
`null` as the port passed to `Server#listen()`. For details see:

nodejs/node#14221

test/instrumentation/modules/restify.js

@@ -39,7 +39,7 @@ test('transaction name', function (t) {
 
   // NOTE: Hostname must be supplied to force IPv4 mode,
   // otherwise this will use IPv6, which fails on Travis CI.
-  server.listen(null, '0.0.0.0', function () {
+  server.listen(0, '0.0.0.0', function () {
     const req = http.get(`${server.url}/hello/world`, res => {
       const chunks = []
       res.on('data', chunks.push.bind(chunks))
@@ -95,7 +95,7 @@ test('error reporting', function (t) {
 
   // NOTE: Hostname must be supplied to force IPv4 mode,
   // otherwise this will use IPv6, which fails on Travis CI.
-  server.listen(null, '0.0.0.0', function () {
+  server.listen(0, '0.0.0.0', function () {
     const req = http.get(`${server.url}/hello/world`, res => {
       res.resume()
       res.on('end', () => {
@@ -147,7 +147,7 @@ test('error reporting from chained handler', function (t) {
 
   // NOTE: Hostname must be supplied to force IPv4 mode,
   // otherwise this will use IPv6, which fails on Travis CI.
-  server.listen(null, '0.0.0.0', function () {
+  server.listen(0, '0.0.0.0', function () {
     const req = http.get(`${server.url}/hello/world`, res => {
       res.resume()
       res.on('end', () => {