This is a follow up to #1364 where go mod was suggested as a better glide replacement than dep. I'll close #1364 in a few minutes in favor of this PR.

Notes:

• I didn't make verbose comments in go.mod as were in glide.yaml because it's not yet clear to me if these will be automatically dropped by go mod.
• go mod doesn't seem to have an option for pruning non-go files.
• Updated Makefile to replace glide calls with go dep calls.

TODO:

• Implement the hack workaround :) suggested by @ericchiang to enable vendoring main packages.
  • Question on that: Where should I write out the file you defined in Migrate to dep #1364, @ericchiang? Would a top level main.go be the right place?
• Fix the CI build when it probably breaks due to my first pass at this.