Fix ActionMailer delivery method configuration

The `delivery_method` configuration line was being appended to an
existing line (which was a comment, thus no error).

*Before*:
```ruby
# config.action_mailer.raise_delivery_errors = false  config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = SMTP_SETTINGS
```

*After*:
```ruby
# config.action_mailer.raise_delivery_errors = false
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = SMTP_SETTINGS
```