Feature: Use Guzzle ^6.3 or ^7.0, to allow API to co-exist with other…

[royarisse]

… (more up to date) packages.

[royarisse]

Upon briefly testing, latest Guzzle seems to work just fine.

[ghermans]

Can this one please get merged asap @arkareth as currently i'm unable to install this package

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Can only install one of: guzzlehttp/guzzle[7.3.0, 6.5.x-dev].
    - Can only install one of: guzzlehttp/guzzle[6.5.x-dev, 7.3.0].
    - Can only install one of: guzzlehttp/guzzle[6.5.x-dev, 7.3.0].
    - openprovider/rest-client-php dev-v1beta requires guzzlehttp/guzzle ^6.2 -> satisfiable by guzzlehttp/guzzle[6.5.x-dev].
    - Installation request for openprovider/rest-client-php dev-v1beta -> satisfiable by openprovider/rest-client-php[dev-v1beta].
    - Installation request for guzzlehttp/guzzle (locked at 7.3.0, required as ^7.3) -> satisfiable by guzzlehttp/guzzle[7.3.0].
    ```