update swiftmailer to 5.2.1

The older versions of swiftmailer have a security problem when using sendmail as the transport. See https://github.com/swiftmailer/swiftmailer/commit/ac8526f5e956c19d6186bf9188a5d8f88f27acbf and http://blog.swiftmailer.org/post/88660759928/security-fix-swiftmailer-5-2-1-released
We cannot fix the swiftmailer version like this. The constraint should be at least `5.2.*` to allow any minor version (like security fixes).