Ability to provide notifications email from env var

Hi, now I have to put email directly in .travis.yml, so when people fork my repo I get email notifications for their builds that I should manually unsubscribe every time. It would be great if I could specify recipients: [$NOTIFICATIONS_EMAIL] and add the email as the env var in settings to avoid that issue.

Notifications are configured when the build requests are recognized, and the environment variables are defined when the build is carried out.

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