Run the vendored fonts tests on OCaml 4.08 and above

This isn't quite the right test, it should be on the version of
ocaml-crunch, but right now it has the behaviour we want.

Author: jonludlam

src/vendor/dune

@@ -1,5 +1,7 @@
 (rule
  (alias runtest)
+ (enabled_if
+  (> %{ocaml_version} 4.08))
  (deps
   (:x odoc_vendor.ml)
   (glob_files *.js)