Updated composer.josn to support Laravel 5.6.

Author: bbrala

composer.json

@@ -5,16 +5,16 @@
     "require": {
         "php": ">=7.0",
         "art4/json-api-client": "^0.9.1",
-        "illuminate/support": "5.3.*|5.4.*|5.5.*",
+        "illuminate/support": "5.3.*|5.4.*|5.5.*|5.6.*",
         "jenssegers/model": "^1.1",
         "php-http/client-implementation": "^1.0",
         "php-http/discovery": "^1.0"
     },
     "require-dev": {
         "fzaninotto/faker": "^1.6",
         "friendsofphp/php-cs-fixer": "^2.0",
-        "graham-campbell/testbench": "^4.0",
-        "phpunit/phpunit": "^6.1",
+        "graham-campbell/testbench": "^4.0|^5.1",
+        "phpunit/phpunit": "^6.1|^7.0",
         "php-http/guzzle6-adapter": "^1.1",
         "php-http/mock-client": "^1.1"
     },