Make mail configurable between SMTP and Mailgun

.rbenv-vars-sample

 # Parameters for sending mail
+# smtp or mailgun
+MAIL_METHOD=
+MAIL_FROM_ADDRESS=
+
+SMTP_SERVER=
+SMTP_USER=
+SMTP_PASS=
+
 MAILGUN_DOMAIN=
 MAILGUN_API_KEY=
-MAIL_FROM_ADDRESS=
 
 # To be used in link construction
 AARDBEI_HOSTNAME=

config/environments/development.rb

   # routes, locales, etc. This feature depends on the listen gem.
   config.file_watcher = ActiveSupport::EventedFileUpdateChecker
 
-  config.action_mailer.delivery_method = :mailgun
+  config.action_mailer.delivery_method = ENV['MAIL_METHOD']
+  config.action_mailer.smtp_settings = {
+    address: ENV['SMTP_SERVER'],
+    user_name: ENV['SMTP_USER'],
+    password: ENV['SMTP_PASS'],
+    authentication: :plain
+  }
   config.action_mailer.mailgun_settings = {
         api_key: ENV['MAILGUN_API_KEY'],
         domain: ENV['MAILGUN_DOMAIN']

config/environments/production.rb

   # Do not dump schema after migrations.
   config.active_record.dump_schema_after_migration = false
 
-  config.action_mailer.delivery_method = :mailgun
+  config.action_mailer.delivery_method = ENV['MAIL_METHOD']
+  config.action_mailer.smtp_settings = {
+    address: ENV['SMTP_SERVER'],
+    user_name: ENV['SMTP_USER'],
+    password: ENV['SMTP_PASS'],
+    authentication: :plain
+  }
   config.action_mailer.mailgun_settings = {
         api_key: ENV['MAILGUN_API_KEY'],
         domain: ENV['MAILGUN_DOMAIN']