Travis: improve the script to install ocamlfind

Necessary from the shift to findlib installation in ade6468a2e6c202
Travis: correct OPAM_VERSION parsing
Fix installation by selecting META file from source tree, not _build
Fix install when only bytecode is compiled by making cmx optional

Also make installation idempotent by removing any previous install
that was around previously.
Travis: test both bytecode-only and bytecode+native installation
Travis: remove a BSD-ism that snuck in (gmake -> make)
Add an `opam` file to test uninstall/install as OPAM package will do
Travis: also attempt a reinstallation to check that upgrade will work
opam: remove the `.opt` versions of the camlp4 binaries as well