Add support for symfony/mailer:^2.x

Problem/Motivation

The module currently supports only symfony/mailer:^1.5. This causes problems when trying to use it in projects that require symfony/mailer:^2.x, as Composer throws a conflict.

Remaining tasks

Update composer.json
Check for any changes needed in the code
Test with both versions of the mailer
Update tests if needed
Tag a new release

User interface changes

None

API changes

None

Data model changes

None