Add opam file

Signed-off-by: John Else <john.else@citrix.com>
travis: switch to new opam 1.2.0 pin based workflow

Use the local opam file to pin the repository and install the
dependencies, rather than hard-coding a list of dependencies.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #18 from johnelse/opam-file

travis: switch to new opam 1.2.0 pin based workflow