Build without godep

Here's a commit to remove dependency on `godep` when **building** Kubernetes. You still use it to manage to Godeps/ tree.

I'm also reverting the Travis change that pulls `godep` into the CI environment, so Travis will let us know that that works for it too.

I also updated the docs to indicate that `godep` is optional and not required for building.

Cheers,
Filipe