Fix start script SMTP parsing

bin/start

@@ -13,7 +13,7 @@ if [ "$DATABASE_URL" ]; then
 fi
 
 if [ "$MAILGUN_SMTP_LOGIN" ]; then
-    export MB_EMAIL_SMTP_HOST="$MAILGUN_SMTP_PASSWORD"
+    export MB_EMAIL_SMTP_HOST="$MAILGUN_SMTP_SERVER"
     export MB_EMAIL_SMTP_PORT="$MAILGUN_SMTP_PORT"
     export MB_EMAIL_SMTP_USERNAME="$MAILGUN_SMTP_LOGIN"
     export MB_EMAIL_SMTP_PASSWORD="$MAILGUN_SMTP_PASSWORD"