Replace Buzz with Guzzle

[Jontsa]

Buzz 0.17 broke backwards compatibility and would have required either a version constraint or code to be updated to match. Here instead code is updated to use Guzzle which has some cool features and is more widely used.

• use Guzzle HTTP client instead of Buzz
• updated tests to match and make use of Guzzle mock handler
• removed carriage returns in some files and fixed formatting
• updated Infobip gateway to use newer JSON API

All tests pass. Sending an actual message was only tested using InfobipGateway.