Fix dependencies for bullet_train <=> bullet_train-api

[jagthedrummer]

Currently we assume that every app will use the -api gem, and we extend parts of the user-facing stuff from the API stuff. But the dependency graph points the wrong way.

We don't have bullet_train-api listed as a dependency in bullet_train.gemspec.

And we do have bullet_train as a dependency in bullet_train-api.gemspec.

We should reverse these and have our declared dependencies match the actual dependency graph.

[jagthedrummer]

Even better would be if we could actually fulfill dependency graph that we currently state. It would be nice if the api gem were optional so that developers could easily replace it with something else.