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

Ideally, we would just use "golint ./..." to check all our our source
files for lint error. However, this does not work because it will
include all packages in the vendor directory. The pull request at:
	https://github.com/golang/lint/pull/325
fixes this issue, so we will use it until the PR has been merged.

This change vendors the source for all our build, formatting, and
linting tools. Generated by running "dep ensure".

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 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

crypto/ssh/terminal gets us information about the terminal, such as
its dimensions and state. It also allows us to manipulate the terminal.
For example, we can put the terminal in raw mode when a passphrase needs
to be entered.

This commit also add the github.com/urfave/cli package which we will use
to write the command line tool component of fscrypt. This tool allows
for us to easily use commands and subcommands.

Note that this is actually the upstream repository with two PRs applied:

https://github.com/urfave/cli/pull/629
https://github.com/urfave/cli/pull/630

They fix bugs in the handling of custom usage errors.

Change-Id: I2d3ba967b1bce8f73440e3a06df4eaba7ab96c19

This commit adds in the two protocol buffer libraries for Go. The
github.com/golang/protobuf/proto package will let Go code read and write
protocol buffers. The github.com/golang/protobuf/jsonpb package lets Go
code output a JSON representation of protocol buffers.

These packages are stored in the vendor directory, meaning that they
will be imported instead of any installed system packages.

Change-Id: I8da8d15864f03a9b3f767a6af18795c8eca64844