Laravel 8 compatibility (mghoneimy#36)

Require guzzle 7 as a dependency to support Laravel 8
- Add dependency on guzzle 7.0.1 alongside guzzle 6
- Add PHP7.4 to travis config

Issue-34: Removed empty braces added in the selection set when no fie…

…lds are selected

- When selection set is empty an empty string is returned
- Tests were modified to have cases with empty selection sets not show empty braces

Opening the possibility of injecting a PSR-18 compatible HTTP Client (m…

…ghoneimy#33)

* Opening the possibility of injecting a PSR-18 compatible HTTP Client

* The use of function stream_for() is discouraged

* Dropping $authorizationHeaders property

* README update

* Providing explanation for removing the headers from the options before creating a guzzle instance

v1.5:

- Added support for running multiple queries under the same Query object
- Added support for running multiple queries under the same QueryBuilder object
- Updated tests to cover new functionalities added
- Added examples in the README.md file

- Add ability to set third parameter in constructor to override Guzzl…

…e httpOptions

- Modified tests to be Windows-friendly

Issue 13: Fixed issue in setting null argument values

Added support for inline fragments in Query and QueryBuilder

v1.2: Add support for variables

GQL-46