Upgrade bosh-cli from v6.4.1 to v6.4.2+

[fnaranjo-vmw]

[#182248334]

bosh-cli was using Gopkg for dependency management until v6.4.1
In v6.4.2 bosh-cli was migrated to use go-modules.

This establishes the foundation to upgrade to bosh-cli/v6

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[fnaranjo-vmw]

Relevant PRs and Issues:

• Allow importing bosh-cli v7 as a dependency bosh-cli#596
• Allow importing bosh-cli v6.4.2 as a dependency bosh-cli#597

[fnaranjo-vmw]

I have built bosh-backup-and-restore CLI with this branch and the problem persists:
failed to check os: ssh.Run failed: ssh.Stream failed: ssh.Dial failed: ssh: handshake failed: ssh: no common algorithm for host key; client offered: [ssh-rsa], server offered: [rsa-sha2-512 rsa-sha2-256 ecdsa-sha2-nistp256 ssh-ed25519]

[fnaranjo-vmw]

The purpose of this PR was mainly to fix an SSH issue which was now solved by: #539

We will create a new PR in the future to make keeping bosh-cli up-to-date possible.