Add more Travis matrix targets (being explicit about CGO)

I've also updated the comments with the current status of the commented-out targets (since we're getting close to having them available for our Travis-use in the relevant upstreams).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #113 from tianon/travis-cgo

Add more Travis matrix targets (being explicit about CGO)