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 updates dependancies to be consistent with Gopkg.toml.
This change was generated by running "dep ensure".

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

This changes the vendored sources of github.com/golang/protobuf,
golang.org/x/crypto/ssh, and golang.org/x/sys/unix to be the current
master versions.

This commit updates the vendored x/sys/unix to the current version. This
means that our vendored version of x/sys/unix no longer contains custom
patches. A few changes are needed in InsertPolicyKey() as the final API
differed slightly from our patches.

Change-Id: Ifff31c04e708f9dc4395ab5bfcc94e42fb46dea5

This commit adds in the golang.org/x/sys/unix package. This package
provides a low-level interface to unix syscalls. We will uses this
package instead of the built-in "syscall" package because the syscall
package is locked down (https://golang.org/pkg/syscall) and is not
exposing any new kernel functionality.

In fact, this is actually a patched version of the x/sys/unix package
pending review (first part: https://go-review.googlesource.com/c/37943).
The version included in this commit exposes all of the filesystem
encryption kernel interfaces to Go code.

Change-Id: Ic5f9c98b858ccb00db97502c9a60e9249aa8ba38