Update Guzzle to 5.0

[dunglas]

To allow using the SDK on project using the latest stable Guzzle version.

[gierschv]

Hi,

Thank you for your contribution, however could you sign your commit (git commit --amend -s --no-edit && git push -f), as required in the CONTRIBUTING guidelines.

Thank you by advance,
Vincent

[dunglas]

@gierschv:

• Added backward compatibility with Guzzle 4
• Signed the commit

[gierschv]

Thanks! LGTM (ping @VincentCasse)

[VincentCasse]

@dunglas Thanks for you pull request
@gierschv thanks for your review :)

Merged