drupal-graphql 3.x updates

[justinlevi]

Travis config, PHPUnit Tests for Create, Update, and Delete.

[justinlevi]

CI tests here: https://travis-ci.org/justinlevi/graphql-examples/builds/352643251

[clemens-tolboom]

• This PR needs a rebase.
• Squash it?
• In trying to update https://www.drupal.org/docs/8/modules/graphql/mutating I had to try this repo.

Both master branch and this PR failed.

./vendor/bin/drush cache-rebuild
Fatal error: Declaration of Drupal\graphql_examples\Plugin\GraphQL\Mutations\UpdateArticle::extractEntityInput(array $args, Youshido\GraphQL\Execution\ResolveInfo $info) must be compatible with Drupal\graphql_core\Plugin\GraphQL\Mutations\Entity\UpdateEntityBase::extractEntityInput($value, array $args, Drupal\graphql\GraphQL\Execution\ResolveContext $context, GraphQL\Type\Definition\ResolveInfo $info) in /.../web/modules/contrib/graphql-examples/src/Plugin/GraphQL/Mutations/UpdateArticle.php on line 25
[error] Drush command terminated abnormally due to an unrecoverable error.
Error: Declaration of Drupal\graphql_examples\Plugin\GraphQL\Mutations\UpdateArticle::extractEntityInput(array $args, Youshido\GraphQL\Execution\ResolveInfo $info) must be compatible with Drupal\graphql_core\Plugin\GraphQL\Mutations\Entity\UpdateEntityBase::extractEntityInput($value, array $args, Drupal\graphql\GraphQL\Execution\ResolveContext $context, GraphQL\Type\Definition\ResolveInfo $info) in /.../web/modules/contrib/graphql-examples/src/Plugin/GraphQL/Mutations/UpdateArticle.php, line 25