travis: enable travis-ci integration
travis: Remove failing version combinations

OCaml 3.xx will not be supported. OCaml 4.01 with opam 1.0 will be
supported in the near future, but that support is blocking on
backporting quickehck to opam 1.0, for which a pull request is already
open in ocaml/opam-repository#1342.
travis: modify CAML_LD_LIBRARY_PATH

In order to run tests without installation, the library needs to know
where dllpacket_stubs.so is located.