travis: remove duplicate line

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

Signed-off-by: John Else <john.else@citrix.com>
travis: Remove handling of OPAM < 1.2

The CI script won't work for earlier versions OPAM anyway, which don't
support pinning to local code repositories.

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