Fix swiftmailer dependency

Remove the "strict" version for swiftmailer dependency.

Example:
- my project use swiftmailer 5.0.2
- i want to use sendgrid (which strictly needs 5.0.1)
- i run into conflicts — i have to downgrade explicitly to 5.0.1

Instead, set your minimal requirement for your vendor. `~5.0.1` means *"I need at least 5.0.1 version, and I'm ok with minor upgrades"*.

Btw, versioning composer.lock for a library is useless.