Allow Rails apps to use Dotenv.overload.

Environment variables are currently only loaded if they are net-new, but we need to be able to update existing environment variables as well. This is necessary for our zero-downtime deployments.

* Moved duplicate calls to configure_spring and set instrumenter to a private method.
* Added paragraph on usage to README.md.
* Added tests for load and overload to test for updates to an existing environment variable.
* Created a context around the load and overload tests and moved common tests into that section.