Update VirtualBox guest additions 5.1.8

scripts/vm-guest-tools.bat

@@ -37,7 +37,7 @@ if exist "C:\Users\vagrant\VBoxGuestAdditions.iso" (
 )
 
 if not exist "C:\Windows\Temp\VBoxGuestAdditions.iso" (
-    powershell -Command "(New-Object System.Net.WebClient).DownloadFile('http://download.virtualbox.org/virtualbox/5.0.26/VBoxGuestAdditions_5.0.26.iso', 'C:\Windows\Temp\VBoxGuestAdditions.iso')" <NUL
+    powershell -Command "(New-Object System.Net.WebClient).DownloadFile('http://download.virtualbox.org/virtualbox/5.1.8/VBoxGuestAdditions_5.1.8.iso', 'C:\Windows\Temp\VBoxGuestAdditions.iso')" <NUL
 )
 
 cmd /c ""C:\Program Files\7-Zip\7z.exe" x C:\Windows\Temp\VBoxGuestAdditions.iso -oC:\Windows\Temp\virtualbox"