Renames newrelic_rails_env to newrelic_env not all ruby apps are rails

I have a rack app build with ruby, it does not make sens for me to add set the `rails_env`. The same applies for my other apps that are not build on top of rails.

Simply renaming the `newrelic_rails_env` variable to `newrelic_env` will make things a lot clearer.

PS: Alternative implementation for #190