Improve initialization of the Railtie

This should fix some loading order related issues within Rails environments.
Going to test this on bladerunner.

@jstorimer @disaacs for review.

This makes sure staging behaves the same in production. I don't think the gem has to be responsible for allowing you to separate between staging and production metrics; there's many ways to do that yourself to the environment in which the library is deployed. E.g., not running an agent, or setting the environment variables differently.