Support symfony/yaml version 5

[viezel]

Since most Laravel packages are locking min req of Symfony packages to v5, then we need to be able to use symfony/yaml v5.

As seen below, we cannot require Vapor-Cli package due to this.

$ composer require laravel/vapor-cli --dev
Using version ^1.6 for laravel/vapor-cli
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: remove symfony/dependency-injection v5.0.5
    - Conclusion: don't install symfony/dependency-injection v5.0.5
    - symfony/yaml 4.2.x-dev conflicts with symfony/dependency-injection[v5.0.5].
    - symfony/yaml 4.3.x-dev conflicts with symfony/dependency-injection[v5.0.5].
    - symfony/yaml v4.2.0 conflicts with symfony/dependency-injection[v5.0.5].
    ...

See https://github.com/symfony/dependency-injection/blob/master/composer.json#L41