Travis CI setup: redundant with Appveyor? need to activate?

[mjambon]

I added a .travis.yml as requested at https://github.com/ocaml-community/meta. There is already an Appveyor config file and because of this, there a was already a "build | passing" icon. Not sure if we should see that icon twice (one for Travis, one for Appveyor) or if something needs to be activated for Travis to kick in (I don't have access to project settings). Or maybe we don't need a Travis config at all since we already have Appveyor. I think the advantage of Appveyor is that it supports Windows. It was set up by @Chris00.

[pmetzger]

I'm having weird trouble getting Travis kicked off. I'll look again in a couple of days, unfortunately I can't devote much time to it now. If I fail to, poke me.

[liyishuai]

Superseded by #79.