Enable mail connection validation for the prod environment.

We can't enable it everywhere because in test and travis environments there is no mail server during
application startup.

Addressed to #413

[ci skip]

Author: php-coder

vagrant/provisioning/roles/mystamps-app/templates/application-prod.properties

@@ -10,6 +10,7 @@ spring.mail.host: smtp.mailgun.org
 spring.mail.port: 25
 spring.mail.username: postmaster@my-stamps.ru
 spring.mail.password: {{ user_mail_password }}
+spring.mail.test-connection: true
 
 spring.messages.cache-seconds: -1
 spring.messages.fallback-to-system-locale: false