Guzzle 7.7 compatibility fixes

[mwolff-fn]

Description

This is a proposal to fix the incompatibility with newer Guzzle versions from 7.7 and Guzzle\Promises from 2.0, as described in this issue:
#48

Changes

To fix "Call to undefined function" error:

• Replace calls to removed function \GuzzleHttp\Promise\rejection_for() with equivalent \GuzzleHttp\Promise\Create::rejectionFor()

To fix failing testsuite:

• Add package colinodell/psr-testlogger as a development dependency
• Replace references to removed class Psr\Log\Test\TestLogger with equivalent ColinODell\PsrTestLogger\TestLogger

[ossycodes]

@mwolff-fn see here #52