gometalinter has been deprecated, with golangci-lint as its spiritual
and recommended successor. Here we switch to using it with an equivalent
configuration, albeit with newer versions of some linters.

To maintain compatibility with existing settings, we have a couple of
things disabled here, specifically:

- tests
        We have a lot of unused code in our tests that choke deadcode.
        We should attempt to clean these up soon so that we can lint our
        testcode.
- govet.check-shadowing = false
        This breaks on redefining `err` which we do all over the nomad
        codebase.