Replace Buzz with HttpClient

[renanbr]

Updates:

• uses $exception->getMessage() when possible in cases we wrap exceptions
• keeps $response variable naming in OAuthConsumer
• branch alias to "dev-master": "5.x-dev"
• bump php dep to >=7.1.3
• remove non supported version from .travis.yml

I don't know why travis is not triggered, btw tests are green on local PHP 7.1.28, 7.2.17 and 7.3.4

[nicolas-grekas]

Continued in #78, thanks @renanbr