Release v4.0.0 Alpha 1

This is the alpha release of OpenTok PHP SDK v4. It primarily features the upgrade of the guzzle dependency from guzzle3 to guzzlehttp6. Although the API does not change in this version, the change has affected every method and has a chance to break SDK users in subtle and unexpected ways which is why the major version number has been incremented.

#132 - Move to Guzzle 6 (#201)

Please use this chance to test your applications and report if you find any problems. If there are no glaring issues the final v4.0.0 will be released in 1 week time.