Version 1.4.0

Adds support for Pure Storage FlashArray REST versions 1.3 and 1.4.

New APIs:

- list_volume_block_differences(self, volume, **kwargs):
  Return a list of block differences for the specified volume and the base volume.
  Requires use of REST API 1.3 or later.

- get_certificate(self, **kwargs):
  Get the attributes of the current array certificate.
  Requires use of REST API 1.3 or later.

- get_certificate_signing_request(self, **kwargs):
  Constructs a certificate signing request (CSR) for signing by a certificate authority (CA).
  Requires use of REST API 1.3 or later.

- set_certificate(self, **kwargs):
  Creates a self-signed certificate or imports a certificate signed by a certificate authority (CA).
  Requires use of REST API 1.3 or later.

Other:

- Fixes a bug where an AttributeError was raised instead of a PureHTTPError when creating a
  FlashArray instance for an array where Purity was not running.