.gitignore

The main sticking point was `config/README.md`, which went into a lot of
detail about each instance. It was quite easy to make a copy and paste
error, as some instances had default URLs and others didn't, some had
fallback instances and others didn't, etc.

This changes a number of things:

1. `config/README.md` now has a table for the Redis instances with a
   single list of instructions that apply to all instances.
2. There are now specs to ensure that each Redis instance has a default
   URL set when no Redis config at all is present.
3. Instances that use a fallback instance for config will automatically
   use the default URL of their fallback instance. They should not
   define their own default URL.
4. The `.gitignore` file does not need to be updated for each new Redis
   instance.
5. The example config files are now gone as `request.yml.example` should
   be sufficient.