+Abella 2.0.2

Also removed abella.2.0.0, which is now obsolete
Abella 2.0.2 requires OCaml 4.01.0+
travis: Fix the check for package availability on older opams

turns out there used to be no good way of simply checking this, so I'm just trying to
install with --dry-run and expecting an empty stdout if the package is unavailable
travis: fix new opam with old compiler availability check
Merge pull request #2951 from dsheets/abella-and-travis

Fix the travis script for new opams with older compilers