Switch the dependency management tool "godep" -> "dep"

[legal90]

tools/godep was deprecated in favor of golang/dep. It removes a lot of hassle with managing dependencies for Go projects. So, I decided to switch to it in this project.

Now direct dependencies are declared in Gopkg.toml and locked in Gopkg.lock. How to update the dependency version:

• Bump it in Gopkg.toml
• run dep ensure
• Done! Don't forget to commit the updated Gopkg.toml, Gopkg.lock and vendor/*

We still have the content of vendor/ dir in the git index for easier start - dep tool is not required to build a project. We can gitignore the vendor/ content in the future if we want.

In the scope of this PR I have also updated some dependencies:

• github.com/docker/machine: 0.12.2 -> 0.16.0
• github.com/hashicorp/go-version: 03c5bf6 -> 1.0.0
• github.com/urfave/cli: 1.18.0 -> 1.20.0 and their transitive dependencies

Acceptance test are passed.

[legal90]

Frankly it's impossible to review such huge changes.

Yeah, I understand :( . The content of vendor/ could be suppressed. Thanks anyway!