Implement new gometalinter capabilities and take care of warnings about them

[rfay]

The Problem/Issue/Bug:

In drud/build-tools#42 @tannerjfco improved our handling of gometalinter quite a lot, and we should use that.

How this PR Solves The Problem:

• Update to build-tools 1.5.1 which has the new gometalinter capabilities
• Handle the warnings it now offers.
• Implement vetshadow which warns about shadowed variables
• gometalinter uses gofmt -s which insists on some simplifications in addition to the normal gofmt behavior. These can of course be calmed with a simple comment.

Key new issues:

1. I changed all imports of testify/assert to be aliased as asrt to solve the shadowing problem of assert vs the import.
2. Change imports of go-homedir from "homedir" alias to "gohomedir" alias for same reason.
3. Since we're now using gometalinter instead of the individual tools, it's easy to use comment directives to calm down a particular tool if it's hitting a false positive (or even something you just don't want to deal with, like removing "dead" code that you don't consider dead.)

Manual Testing Instructions:

You may want to look at the changes commit-by-commit so there's not just a whole pile of changes.

Automated Testing Overview:

Related Issue Link(s):

drud/build-tools#42 was the build-tools upgrade for gometalinter.

Release/Deployment notes: