remove CamlinternalPr for compilation with OCaml 4.02.0

See issue #572.

Batteries doesn't compile with OCaml 4.02.0 because Printf.CamlinternalPr is gone.
I don't know what CamlinternalPr is used for, but it doesn't appear anywhere else in the code and the comment says it shouldn't be called.
It would probably be better to keep it for versions < 4.02.0, but [build/prefilter.ml](../blob/master/build/prefilter.ml) only allows filtering for a minimum version. Maybe it makes sense to allow specifying a comparison operator like `##V<4.2##` (where `##Vx##` will default to `##V>=x##`).