Fix deprecated DefinitionDecorator for Symfony 4

[renatoaraujo]

This fix the issue #57

[l3ackslash0]

Hello,

Has it been merged to dev-master?

[renatoaraujo]

@jblondeau2 there is already a PR open for this issue. See #55

Workaround: Fork and fix into your repository and add to your composer.json:

"repositories": [
{
"type": "vcs",
"url": "https://github.com/renatoaraujo/swiftmailer-mailgun-bundle"
}
],

Hope this help you.

Cheers!