Replace Buzz with Guzzle#23

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Jontsa wants to merge 3 commits intoxi-project:masterfrom

Jontsa:guzzle-http

Commits on Mar 30, 2018

use Guzzle HTTP client instead of Buzz, updated tests, added new tests, removed carriage returns in some files and fixed formatting
Jontsa
committed
bumped minimum PHP version to 5.5 which is required by Guzzle 6 and traits
Jontsa
committed
updated InfoBip gateway to use JSON API, updated code to use short array syntax
Jontsa
committed