travis: install OUnit

Signed-off-by: John Else <john.else@citrix.com>
travis: use new opam syntax

Signed-off-by: John Else <john.else@citrix.com>
Don't call oasis from the Makefile

Signed-off-by: John Else <john.else@citrix.com>
Add opam file

Signed-off-by: John Else <john.else@citrix.com>
travis: use opam file to install deps

Signed-off-by: John Else <john.else@citrix.com>
travis: Install deps with installext

Signed-off-by: John Else <john.else@citrix.com>