[AppVeyor] upgrade OCaml from 4.02.3 to 4.07.0

appveyor.yml

@@ -42,7 +42,7 @@ install:
       )
     - 7z x "opam.tar.xz" -so | 7z x -aoa -si -ttar
     - '%CYG_ROOT%/bin/bash -lc "cd \"$OLDPWD\" && bash opam${ARCH}/install.sh"'
-    - '%CYG_ROOT%/bin/bash -lc "opam init mingw \"https://github.com/fdopen/opam-repository-mingw.git\" --comp 4.02.3+mingw${ARCH}c --switch 4.02.3+mingw${ARCH}c --auto-setup --yes"'
+    - '%CYG_ROOT%/bin/bash -lc "opam init mingw \"https://github.com/fdopen/opam-repository-mingw.git\" --comp 4.07.0+mingw${ARCH}c --switch 4.07.0+mingw${ARCH}c --auto-setup --yes"'
     - '%CYG_ROOT%/bin/bash -lc "opam update --yes"'
     - '%CYG_ROOT%/bin/bash -lc "opam pin add haxe \"$APPVEYOR_BUILD_FOLDER\" --no-action --yes"'
     - '%CYG_ROOT%/bin/bash -lc "opam install haxe --deps-only --yes"'