[AppVeyor] cache the ocaml installer to avoid network issues

appveyor.yml

@@ -18,10 +18,15 @@ services:
 
 skip_tags: true
 
+cache:
+    - ocaml-installer.exe -> appveyor.yml
+
 install:
     - 'git submodule update --init --recursive'
     # Install ocaml
-    - curl -fsS -o ocaml-installer.exe --retry 3 http://gallium.inria.fr/~protzenk/caml-installer/ocaml-4.02.3-i686-mingw64-installer4-opam.exe
+    - if not exist "ocaml-installer.exe" (
+        curl -fsS -o ocaml-installer.exe --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'