Upgraded to Go 1.9 and dep

[febbraro]

Move away from Godep to dep (thanks to @sdboyer)

This is for #68

[grayside]

I don't see travis updates. There are hidden godep steps there, as well as an env var related to vendoring.

[grayside]

Not sure where it belongs, but depending readme recommends ci should use a fixed version.

[grayside]

dep readme, not depending

[febbraro]

@grayside Updated based on feedback. Multiple dependencies did not have explicit versions, so I pinned to a revision

[sdboyer]

hola friends! couple quick notes on this:

1. you'll probably have an easier time if you use release versions of dep instead of tip.
2. if there's ever a possibility that something else will depend on rig, you'll likely want to place the Gopkg.{toml,lock},vendor at the repository root.
3. in general, you needn't specify a revision in Gopkg.toml - instead, trust Gopkg.lock to pin things appropriately. if there's no tags to consume, prefer a branch. then just make sure to always use dep ensure -update <deps...> instead of dep ensure -update (globally), and those pinned versions won't change.

[grayside]

Thanks @sdboyer ! (1) sounds good, (2) we decided to do in a follow-up if only for standards & practices, (3) Aha, that makes sense from npm/composer/etc.

[febbraro]

Thanks @sdboyer! Appreciate the feedback

[febbraro]

Moving all this work to #95