Improve Travis scripts to pin/install an upstream dep
Simplify build system to not use `ocamlbuild`.
Travis: Only test on 4.00+
Merge pull request #18 from avsm/master

Simplify build system to not use `ocamlbuild`.
Merge branch 'master' of https://github.com/mirage/mirage-types
[make] remove build shell script; complete Makefile