Add opam file

Signed-off-by: David Scott <dave.scott@citrix.com>
Use opam 1.2 for travis

Signed-off-by: David Scott <dave.scott@citrix.com>
We need a modern cstruct

Signed-off-by: David Scott <dave.scott@citrix.com>
opam: use 'opam config -env'

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #11 from djs55/opam-1.2

Use opam 1.2