Future proof rollbar config so that it is already enabled when we create a staging environment

It is safe to keep this around, rails won't error if the environment is not defined

```
[1] pry(main)> Rails.env.development?
=> true
[2] pry(main)> Rails.env.staging?
=> false
[3] pry(main)> Rails.env.foobar?
=> false
```