Consistently advertise opam-lib package in opam and ocamlfind
Travis: install opam with apt-get in test mode
Safeguard: fail on empty solution unless --no-base-packages
Merge pull request #1560 from OCamlPro/fix-install-travis-safeguard

Fix opam-lib package name at 'make libinstall', new travis tests; safeguard on removing everything.