travis: remove Laravel 5.6 and 5.7 from the test matrix

- 5.6: bug fixes ended on August 7th, 2018 and security fixes on February 7th, 2019
- 5.7: bug fixes ended on March 4th, 2019 and security fies on September 4th, 2019

Also: there's some weird edge case bug with 5.7 in the test \Rebing\GraphQL\Tests\Unit\SchemaHyphenInPathTest::testWithHyphen which makes it fail, see https://travis-ci.org/rebing/graphql-laravel/jobs/579289418 (this build was previously green until I restarted it recently):
```
1) Rebing\GraphQL\Tests\Unit\SchemaHyphenInPathTest::testWithHyphen

Failed asserting that exception of type "ErrorException" matches expected exception "Symfony\Component\HttpKernel\Exception\NotFoundHttpException". Message was: "Invalid argument supplied for foreach()" at

/home/travis/build/rebing/graphql-laravel/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:739

/home/travis/build/rebing/graphql-laravel/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:718

/home/travis/build/rebing/graphql-laravel/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:690

/home/travis/build/rebing/graphql-laravel/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:756

/home/travis/build/rebing/graphql-laravel/tests/Unit/SchemaHyphenInPathTest.php:29
```
Locally everything is working fine so I don't see a reason to pursue this further.

Author: mfn

CHANGELOG.md

@@ -4,7 +4,7 @@ CHANGELOG
 [Next release](https://github.com/rebing/graphql-laravel/compare/2.1.0...master)
 --------------
 ### Changed
-- Forward PHP engine errors to the application error handler
+- Forward PHP engine errors to the application error handler [\487 / mfn](https://github.com/rebing/graphql-laravel/pull/487)
 
 2019-08-27, 2.1.0
 -----------------

tests/Unit/EngineErrorInResolverTests/EngineErrorInResolverTest.php

@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rebing\GraphQL\Tests\Unit\EngineErrorInResolverTests;
+
+use Mockery;
+use Rebing\GraphQL\Tests\TestCase;
+use Illuminate\Contracts\Debug\ExceptionHandler;
+use Symfony\Component\Debug\Exception\FatalThrowableError;
+
+class EngineErrorInResolverTest extends TestCase
+{
+    private const ERROR_REGEX = '/QueryWithEngineErrorInCodeQuery::getResult\(\) must be of the type int, string given, called in/';
+
+    public function testForEngineError(): void
+    {
+        $result = $this->graphql('query { queryWithEngineErrorInCode }', [
+            'expectErrors' => true,
+        ]);
+
+        $this->assertRegExp(static::ERROR_REGEX, $result['errors'][0]['debugMessage']);
+    }
+
+    protected function resolveApplicationExceptionHandler($app)
+    {
+        // We expect the error in QueryWithEngineErrorInCodeQuery  to trigger
+        // reporting to the handler (as opposed to swallowing it silently).
+        $handlerMock = Mockery::mock(ExceptionHandler::class);
+        $handlerMock
+            ->shouldReceive('report')
+            ->with(Mockery::on(
+                function (FatalThrowableError $error) {
+                    $this->assertRegExp(static::ERROR_REGEX, $error->getMessage());
+
+                    return true;
+                }
+            ))
+            ->once();
+
+        $app->instance(ExceptionHandler::class, $handlerMock);
+    }
+
+    protected function getEnvironmentSetUp($app)
+    {
+        parent::getEnvironmentSetUp($app);
+
+        $app['config']->set('graphql.schemas.default', [
+            'query' => [
+                QueryWithEngineErrorInCodeQuery::class,
+            ],
+        ]);
+    }
+}

tests/Unit/EngineErrorInResolverTests/QueryWithEngineErrorInCodeQuery.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rebing\GraphQL\Tests\Unit\EngineErrorInResolverTests;
+
+use GraphQL\Type\Definition\Type;
+use Rebing\GraphQL\Support\Mutation;
+
+class QueryWithEngineErrorInCodeQuery extends Mutation
+{
+    protected $attributes = [
+        'name' => 'queryWithEngineErrorInCode',
+    ];
+
+    public function type(): Type
+    {
+        return Type::nonNull(Type::string());
+    }
+
+    public function resolve($root, $args): string
+    {
+        // This code deliberately creates a PHP error!
+        return $this->getResult('result');
+    }
+
+    private function getResult(int $string): string
+    {
+    }
+}