If no Heap.app_id is specified, the library should fail gracefully

[jeffling]

This is really not a big deal but here goes.

In certain environments like testing, the library should not fail with an error and crash the application.

Look at stripe's example:

No stripe.com API key was configured for environment test! this application will be
unable to interact with stripe.com. You can set your API key with either the environment
variable `STRIPE_SECRET_KEY` (recommended) or by setting `config.stripe.secret_key` in your
environment file directly.

It doesn't fail, and educates the user to configure the environment better.