support Laravel 5.6 (#525)

Unlocking Laravel 5.6 doesn't seem to break anything.

Author: PawelDmochowski

composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "php": ">=7.0.0",
-        "laravel/framework": "~5.2||~5.3||~5.4||~5.5"
+        "laravel/framework": "~5.2||~5.3||~5.4||~5.5||~5.6"
     },
     "require-dev": {
         "orchestra/testbench-browser-kit": "~3.4",