Add AWS IAM authorization support (#64)

InlineFragment: read selectionset from QueryBuilderInterface (#58)

Support guzzlehttp/psr7 2+, remove support for guzzlehttp/psr7 < 1.7 (#…

…61)

Added support for MutationBuilder

- Added MutationBuilder class
- Added MutationBuilderTest class to cover new logic
- Removed EmptySelectionSetException being thrown from AbstractQueryBuilder when selection set is empty

Prevented users from using "GET" method

- Throwing exception if client is constructed with request method other than "POST"
- Formatted code
- Improved code coverage

Issue-49: Modified string formatting logic to account for string valu…

…es starting with '$' that are not variables

Added support for PHP8

Added support for setting query field aliases

Laravel 8 compatibility (#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