add laravel 5.7 compatibility and phpunit 7 (#133)

Author: fgreinus

composer.json

@@ -29,13 +29,13 @@
     "type": "project",
     "require": {
         "php": ">=5.5.9",
-        "illuminate/support": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*",
+        "illuminate/support": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*|5.7.*",
         "webonyx/graphql-php": "~0.10.0",
-        "laravel/framework": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*"
+        "laravel/framework": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*|5.7.*"
     },
     "require-dev": {
         "orchestra/testbench": "3.1.*|3.2.*|3.3.*|3.4.*|3.5.*",
-        "phpunit/phpunit": "^5.5|~6.0"
+        "phpunit/phpunit": "^5.5|~6.0|~7.0"
     },
     "autoload": {
         "psr-0": {