No guard for DATABASE_URL env var

[osis]

While setting up a new environment, I noticed I started getting this error...

Test Panicked
  Error 1251: Client does not support authentication protocol requested by server; consider upgrading MariaDB client
  /usr/local/go/src/runtime/panic.go:491

Which was a bit alarming, but I think it's actually the result of not setting this environment variable.

Tracked it down to maybe this part of the source not functioning properly?

Also noticed that the tests don't cover this case. https://github.com/cloudfoundry-incubator/notifications/blob/master/application/environment_test.go#L63

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[osis]

Turns out it was that was surfacing this configuration issue.

mysqljs/mysql#1507 (comment)

The error that you eventually cascades down to...

Test Panicked
  Error 1049: Unknown database 'notifications_test'

still, a "DOCKER_URL not set"-like error would be a nicer experience.