Move repo dependency management from go dep to modules

[leighmcculloch]

This repository uses dep to manage it's dependencies and the Go tooling and the community has shifted to use Modules. Modules were automatically enabled in Go 1.11+ when used in repositories outside a GOPATH, and will be always enabled in Go 1.13 that is currently in rc1 as of August 21st.

We should move to Modules to remain consistent with the wider Go community, and to make it easier for developers contributing or depending on packages in this repository to use common tooling.