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 commit adds in the golang.org/x/crypto/hkdf package which contains
the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as
defined in RFC 5869.

This package will be used later in the crypto package to stretch a
single cryptographic key into multiple keys.

Change-Id: I387230344bdeadc7d38c46994a228eecda2e12bd