travis: PACKAGE=tar-mirage OCAML_VERSION=4.03
travis: PACKAGE=tar-mirage OCAML_VERSION=4.03

Previously we didn't specify the PACKAGE so the scripts looked for
and failed to find an `opam` file. The `tar-mirage` package builds
enough for the tests to run.

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

mirage/ocaml-tar