Fix Rakefile and deploy notification to Errbit

- Remove the remaining exception email config
- Make Rake available in production, and make the Rakefile only require rspec in not-production
- Specify Airbrake config in Ruby directly, rather than expecting to have to parse YAML
- Don't include Airbrake's built-in Rake tasks - we don't need any of them
- Add a custom Rake task to notify Errbit of a deploy, based on the similar one in Airbrake

Related alphagov-deployment PR coming in a minute, which will need to be merged first...