Use `bundle exec` with `rake`

Update README.md to use `bundle exec` when executing rake to prevent errors due to incompatible versions of rake.
Prevent layout blowing up with no logged in user

This currently causes the failure page to blow up because current_user
is nil.
Add integration test for layout behaviour when signed out
Add in a step sequence for getting the app running

- It took me a while to get up and running so I thought I'd add some copy and paste love to the readme