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).
Actually, we can't (not for ARM); see https://code.google.com/p/go/issues/detail?id=1880#c18 for where Dave mentions that cross-compiling ARM with CGO support enabled is a fools errand (and he's one of the guys who'd be crazy enough to want to make that work).
> If you need cgo on arm, you will have to compile on arm directly.