Drop support for PHP < 7.2 and improve test matrix (#868)

* travis: use explicit test matrix and drop PHP < 7.2

* composer: use more liberal version requirements for orchestra

This is to easier satisfy when we want to use older Laravel versions on travis

* tests: add shim for assertStringContainsString for older Laravel/phpunit versions

Specifically, Laravel 5.5 works with an older phpunit version, which
does not feature assertStringContainsString

* composer: bump minimum PHP version to 7.2

* travis: explicitly install Mockery for testing older Laravel versions

* composer: make mockery a root requirement and remove the one from travis

* travis: further improve based on suggestions from https://github.com/fruitcake/laravel-cors/blob/master/.travis.yml

* tests: skip test if mixin markers are already present in model

We're also testing Laravel 5.5 which contains them (they were removed later)

composer.json

@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "php": ">=7",
+        "php": ">=7.2",
         "illuminate/support": "^5.5|^6",
         "illuminate/console": "^5.5|^6",
         "illuminate/filesystem": "^5.5|^6",
@@ -23,7 +23,8 @@
         "illuminate/view": "^5.5|^6",
         "phpro/grumphp": "^0.14",
         "squizlabs/php_codesniffer": "^3",
-        "orchestra/testbench": "^4.4"
+        "orchestra/testbench": "^3|^4",
+        "mockery/mockery": "^1.3"
     },
     "autoload": {
         "psr-4": {