appveyor: Update winflexbison download URL.
This particular version got moved into a `old_versions` subdirectory.
diff --git a/appveyor.yml b/appveyor.yml
index c2efa7e..6e69cbf 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -55,7 +55,7 @@
- python -m pip install --egg scons==2.4.1
- scons --version
# Install flex/bison
-- if not exist "%WINFLEXBISON_ARCHIVE%" appveyor DownloadFile "http://downloads.sourceforge.net/project/winflexbison/%WINFLEXBISON_ARCHIVE%"
+- if not exist "%WINFLEXBISON_ARCHIVE%" appveyor DownloadFile "https://downloads.sourceforge.net/project/winflexbison/old_versions/%WINFLEXBISON_ARCHIVE%"
- 7z x -y -owinflexbison\ "%WINFLEXBISON_ARCHIVE%" > nul
- set Path=%CD%\winflexbison;%Path%
- win_flex --version