Give environmental variables precedence when sending emails over the galaxy config

The commit https://github.com/meteor/meteor/commit/6db81808fad88d30dbbe22729b6aac835564910c broke the ability to set a custom SMTP server with meteor deploy hosting and presumably galaxy hosted apps too:

The section of the docs goes

> For apps deployed with meteor deploy, MAIL_URL defaults to an account (provided by Mailgun) which allows  apps to send up to 200 emails per day; you may override this default by assigning to process.env.MAIL_URL before your first call to Email.send.

So here the precedence is just re-ordered and environmental variables take precedence over a galaxy configuration.