Merge pull request ember-fastboot#153 from bekzod/let-const

`var` => `const`

src/fastboot-info.js

@@ -1,8 +1,8 @@
 'use strict';
 
-var RSVP = require('rsvp');
-var FastBootRequest = require('./fastboot-request');
-var FastBootResponse = require('./fastboot-response');
+const RSVP = require('rsvp');
+const FastBootRequest = require('./fastboot-request');
+const FastBootResponse = require('./fastboot-response');
 
 /*
  * A class that encapsulates information about the

src/fastboot-request.js

@@ -1,7 +1,7 @@
 'use strict';
 
-var cookie = require('cookie');
-var FastBootHeaders = require('./fastboot-headers');
+const cookie = require('cookie');
+const FastBootHeaders = require('./fastboot-headers');
 
 class FastBootRequest {
   constructor(request, hostWhitelist) {

src/fastboot-response.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var FastBootHeaders = require('./fastboot-headers');
+const FastBootHeaders = require('./fastboot-headers');
 
 class FastbootResponse {
   constructor(response) {

src/install-source-map-support.js

@@ -1,4 +1,4 @@
-var sourceMapSupport = require('source-map-support');
+const sourceMapSupport = require('source-map-support');
 
 function prepareStackTrace(error, stack) {
   return error + stack.map(function(frame) {

src/sandbox.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var chalk = require('chalk');
+const chalk = require('chalk');
 
 class Sandbox {
 

src/vm-sandbox.js

@@ -1,7 +1,7 @@
 'use strict';
 
-var vm = require('vm');
-var Sandbox = require('./sandbox');
+const vm = require('vm');
+const Sandbox = require('./sandbox');
 
 class VMSandbox extends Sandbox {
   constructor(options) {