+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
cow.0.8.1: restrict to ocaml < 4.02 per mirage/ocaml-cow#48
Remove unstable mirage-www package from the main repo
Merge pull request #2949 from samoht/mirage-www

Remove unstable mirage-www package from the main repo
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
If you skip, do it on a log.

but don't fall into the river.

of lava.
Merge pull request #2952 from dsheets/abella-and-travis

Travis: Skipping an install should print the reason for skipping
Merge pull request #2944 from dsheets/cow_0_8_1_ocaml_version

cow.0.8.1: restrict to ocaml < 4.02 per mirage/ocaml-cow#48
add minimal version constraints for irmin
+libres3 v0.9
Remove unstable ocaml-git versions
Merge pull request #2953 from chambart/patch-7

add minimal version constraints for irmin
Merge pull request #2954 from edwintorok/libres3_v0.9

+libres3 v0.9
Merge pull request #2955 from samoht/git

Remove unstable ocaml-git versions