[AppVeyor] use curl to download such that it is more robust to network issues

appveyor.yml

@@ -21,7 +21,7 @@ skip_tags: true
 install:
     - 'git submodule update --init --recursive'
     # Install ocaml
-    - appveyor DownloadFile "http://gallium.inria.fr/~protzenk/caml-installer/ocaml-4.02.3-i686-mingw64-installer4-opam.exe" -FileName "ocaml-installer.exe"
+    - curl -fsS -o ocaml-installer.exe --max-time 120 --retry 3 http://gallium.inria.fr/~protzenk/caml-installer/ocaml-4.02.3-i686-mingw64-installer4-opam.exe
     - ocaml-installer.exe /S
     - 'C:\Users\Public\Desktop\cygwin-setup.exe -g -q -R "%CYG_ROOT%" -P make -P git -P mingw64-i686-zlib'
     - 'set PATH=%PATH%;%CYG_ROOT%/usr/i686-w64-mingw32/sys-root/mingw/bin'