FatalThrowableError in MailgunTransport.php line 67: Undefined class constant 'VERSION'

I'm getting this error when trying to send email from within my Laravel 5.1 application. This function was working quite well for a very long time, but I stopped using my application for about a year and now when I try to use it I'm getting this error. I don't think I've made any changes to anything related to emails, so I'm not sure what the cause could be.

Here is what my composer.json looks like:

    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "php": ">=5.5.9",
        "laravel/framework": "5.1.*",
        "barryvdh/laravel-ide-helper": "^2.1",
        "laravelcollective/html": "5.1.*",
        "nesbot/carbon": "1.39.*",
        "intervention/image": "^2.3",
        "fzaninotto/faker": "^1.7",
        "bugsnag/bugsnag-laravel": "^2.0",
        "silber/bouncer": " v1.0.0-rc.4",
        "doctrine/dbal": "v2.9",
        "spatie/laravel-medialibrary": "^4.0.0",
        "rap2hpoutre/laravel-log-viewer": "^2.2"
    },
    "require-dev": {
        "mockery/mockery": "0.9.*",
        "phpunit/phpunit": "~4.0",
        "phpspec/phpspec": "~2.1"
    },

PHP v7.3

via Chebli Mohamed