As the Go community transitions to using the modules ecosystem,
we want to only support one way of managing dependencies.
So this change moves to only using Go modules for dependency management.

This means that our effective minimum Go version increases to Go 1.11.
To account for this, we also update:
  - the documentation
  - Makefile
  - CI scripts

This change transitions to dep, sets up Gopkg.toml, and purnes the
maximum allowable number of files.

This commit adds in the github.com/pkg/errors package for handling
errors. This allows us to continue to add context to our errors as they
move up the call stack, but still be able to switch on their root cause.

Change-Id: I6fd55f184318ea1e28c93a84444a2627bff03682