Update to Laravel 6.x and PHP 7.2/7.3

[jason-klein]

This health-check library depends on the phpsafari/typed-collections (https://github.com/OveD-php/typed-collections) library. I have updated both libraries to support Laravel 6.x and I am submitting PR for both libraries.

Both PRs should be tagged/released at the same time, since this PR expect the phpsafari/typed-collections library namespace to change from "Phpsafari" to "PhpSafari".

Both of my PRs drop support for Laravel 5.x due to dependency incompatibilities. My suggestion is to align the versioning of both of these Laravel libraries with Laravel versioning so that the existing versions (0.0.7 and 2.0.2) remain available as-is (or as 5.0.0?) to Laravel 5.x users and the new versions become available as 6.0.0 for Laravel 6.x users.

This library PR is currently forced to use my patched 6.0.0 version of phpsafari/typed-collections. After you upgrade the phpsafari/typed-collections library to Laravel 6.x, be sure to remove the "repositories" section from composer.json in this library.

Here is my PR for phpsafari/typed-collections with Laravel 6.x upgrades: OveD-php/typed-collections#3

If you would like to test my updates with Laravel 6.x before accepting this PR and before publishing updated library versions, you can follow these steps:

1. Add the following code to your composer.json file:

"repositories": [
       {
           "type": "package",
           "package": {
               "name": "vistik/typed-collections",
               "version": "6.0.0",
               "source": {
                   "url": "https://github.com/jason-klein/typed-collections",
                   "type": "git",
                   "reference": "pr-laravel6x"
               },
               "type": "library",
               "autoload": {
                   "psr-4": {
                       "PhpSafari\\": "src/"
                   },
                   "files": [
                       "src/helpers.php"
                   ]
               }
           }
       },
     {
         "type": "package",
         "package": {
             "name": "phpsafari/health-checks",
             "version": "6.0.0",
             "source": {
                 "url": "https://github.com/jason-klein/health-checks",
                 "type": "git",
                 "reference": "pr-laravel6x"
             },
             "type": "library",
             "autoload": {
                 "psr-4": {
                     "PhpSafari\\": "src/"
                 }
             }
         }
     }
 ],

2. Tell composer to require the 6.0.0 version of each package, which will read version 6.0.0 from my repo until a newer version is published. When you publish newer updates of each package (e.g. 6.0.1), composer should automatically receive those updates directly from your official repo even if you forget to remove the custom repositories entries above.

composer require phpsafari/health-checks ^6.0 vistik/typed-collections ^6.0

The 2-step method above is untested, but very similar to the method I am successfully using to import the custom versions of the libraries from my dev- branches into Laravel 6.

If you do not move to 6.0.0 versioning after using the method above, you will need to uninstall each package (e.g. composer remove), remove the custom repositories entries from your composer.json, then reinstall each package (e.g. composer require).