Use fewer customizations to RSpec configuration

* These were tangentially introduced in 8718a47 while adding WebMock.
* `fail_fast` is a personal preference which should live in `~/.rspec`.
* `infer_base_class_for_anonymous_controllers` is true in RSpec 3.

This came up while discussing thoughtbot/learn#615.

https://github.com/thoughtbot/learn/pull/615#discussion_r9708534