Coinbase PHP 2.9.0

This is a maintenance release.

• Update Guzzle to latest version
• Update PHPUnit to latest version
• Update SSL certificate
• Update tests to work properly with latest PHPUnit version
• Fix issue in getTimestamp function
• Remove Orders API
• Remove Checkouts API
• Remove Merchant API

Note: The removed APIs are no longer available in the Coinbase API, they have been moved to Coinbase Commerce. There is an official repository for the Coinbase Commerce PHP Client.