Update hashes in Windows build script

Boxes/.gitignore

@@ -1,2 +0,0 @@
-*
-!.gitignore

build.ps1

@@ -174,15 +174,14 @@ function list_providers {
 function download_boxes {
   Write-Verbose '[download_boxes] Running..'
   if ($PackerProvider -eq 'virtualbox') {
-    $win10Hash = '30b06e30b36b02ccf1dc5c04017654aa'
-    $win2016Hash = '614f984c82b51471b5bb753940b59d38'
+    $win10Hash = 'd6304f01caa553a18022ea7b5a73ad0d'
+    $win2016Hash = 'b59cf23dfbcdb63c0dc8a98fbc564451'
   }
   if ($PackerProvider -eq 'vmware') {
-    $win10Hash = '174ad0f0fd2089ff74a880c6dadac74c'
-    $win2016Hash = '1511b9dc942c69c2cc5a8dc471fa8865'
+    $win10Hash = '4355e9758a862a6f6349e31fdc3a6078'
+    $win2016Hash = '249fc2472849582d8b736cdabaf0eceb'
   }
 
-  
   $win10Filename = "windows_10_$PackerProvider.box"
   $win2016Filename = "windows_2016_$PackerProvider.box"
 
@@ -241,8 +240,7 @@ function preflight_checks {
 
   Write-Verbose '[preflight_checks] Checking for pre-existing boxes..'
   if ((Get-ChildItem "$DL_DIR\Boxes\*.box").Count -gt 0) {
-    Write-Error 'You appear to have already built at least one box using Packer. This script does not support pre-built boxes. Please either delete the existing boxes or follow the build steps in the README to continue.'
-    break
+    Write-Host 'You seem to have at least one .box file present in the Boxes directory already. If you would like fresh boxes downloaded, please remove all files from the Boxes directory and re-run this script.'
   }
 
   # Check to see that no vagrant instances exist