Upgrade PhpUnit and required libs.

Author: nicekiwi

composer.json

@@ -1,6 +1,6 @@
 {
   "name": "syntax/steam-api",
-  "description": "A steam-api client for Laravel 5.",
+  "description": "A steam-api client for Laravel 5.5+",
   "license": "MIT",
   "authors": [
     {
@@ -9,15 +9,15 @@
     }
   ],
   "require": {
-    "php": ">=5.5.0",
-    "laravel/framework": "^5.0",
+    "php": "^7.0.0",
+    "laravel/framework": "^5.5.0",
     "guzzlehttp/guzzle": "^6.0",
     "nukacode/database": "^1.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "~3.0",
-    "mockery/mockery": "0.9.3",
-    "orchestra/testbench": "~3.0"
+    "phpunit/phpunit": ">=6.0",
+    "orchestra/testbench": "^3.0",
+    "vlucas/phpdotenv": "^3.3"
   },
   "autoload": {
     "psr-4": {