This PR upgrades the SDK to use Guzzle 6.

I am still exercising these changes with functionality very similar to the examples (shell.php and task.php), hence the "hold" designation for now.

I am curious how others are using this SDK now or would like to in the future.

My initial use case is seeding vaults in developer and CI environments with secrets similar to the deployed environments.

This will likely be a small standalone application and/or repository (vs being called within another app). Vault is already installed, configured, and running on the host via configuration management tool.

The examples already in the project do this vault seeding sequence very well for CakePHP apps, but I think there is an opportunity to implement some of this logic on a Jippi\Vault\Helper namespace very similar to the Consul SDK and be usable within any app or framework.

A "Seed" helper with some convenience methods fits my current use case well:

        $this->reset();

        $this->create();
        $this->unseal();

        $this->Vault->waitFor(['sealed' => false, 'standby' => false]);
        $this->Vault->clear();

        $this->mounts();

But there may be a better name or context than "seed" from which to offer these methods.