Fix MikTeX download link

tools/appveyor/install.ps1

@@ -13,8 +13,8 @@ try {
 
     Write-Host -ForegroundColor Green "`n### Installing Miktex ###`n"
     if (-not (Test-Path "c:\miktex\texmfs\install\miktex\bin\pdflatex.exe")) {
-        & appveyor DownloadFile https://ftp.uni-erlangen.de/mirrors/CTAN/systems/win32/miktex/setup/windows-x86/miktex-portable.exe
-        & 7z x miktex-portable.exe -oc:\miktex -bso0 -bsp0
+        & appveyor DownloadFile https://ftp.uni-erlangen.de/mirrors/CTAN/systems/win32/miktex/setup/windows-x86/miktex-portable-2.9.6753.exe
+        & 7z x miktex-portable-2.9.6753.exe -oc:\miktex -bso0 -bsp0
 
         # Remove some big files to reduce size to be cached
         Remove-Item -Path c:\miktex\texmfs\install\doc -Recurse
@@ -60,4 +60,4 @@ try {
     # Wait a bit to make sure appveyor shows the error message
     Start-Sleep 10
     throw
-}
+}