Add support for Laravel 5.8, and dropped support for Laravel below than 5.5

Author: richan-fongdasen

composer.json

@@ -21,16 +21,16 @@
         "source": "https://github.com/richan-fongdasen/laravel-varnishable"
     },
     "require": {
-        "php": "^5.5.9 || ^7.0 || ^7.1.3",
+        "php": "^7.1.3",
         "guzzlehttp/guzzle": "~6.0",
-        "illuminate/database": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*|5.7.*",
-        "illuminate/support": "5.1.*|5.2.*|5.3.*|5.4.*|5.5.*|5.6.*|5.7.*"
+        "illuminate/database": "5.5.*|5.6.*|5.7.*|5.8.*",
+        "illuminate/support": "5.5.*|5.6.*|5.7.*|5.8.*"
     },
     "require-dev": {
-        "mockery/mockery": "~0.9.8",
-        "orchestra/database": "~3.1",
-        "orchestra/testbench": "~3.1",
-        "phpunit/phpunit": "~7.0 || ~6.0 || ~5.2 || ~4.8"
+        "mockery/mockery": "~1.0",
+        "orchestra/database": "~3.5",
+        "orchestra/testbench": "~3.5",
+        "phpunit/phpunit": "^8.0 || ~7.0 || ~6.0"
     },
     "config": {
         "sort-packages": true