Add X-GitHub-Services-Version to the service request headers

This exposes the currently running version of github/github-services as a request header. This is available for all `http` based services as well as webhooks.

/cc #906 @kdaigle @atmos
> This sha doesn't match line 89 below, is this intentional?

It is truncated to the first 8 characters.

> What's this for vs using Service?

`TestService` is the subclass of `Service` used in this particular file. See [here](https://github.com/github/github-services/blob/expose-version-sha-in-headers/test/service_test.rb#L6-L9) and [here](https://github.com/github/github-services/blob/expose-version-sha-in-headers/test/service_test.rb#L103-L105).