Download the app protoype from github instead of embedding it.

- Fetch the latest tag from carbonfive/raygun-rails4.
- Check to see if the tarball is already cached under ~/.raygun.
- Download if it's not cached.
- Check that the raygun gem is new enough for the protoype.
- Generate the new app using the newest tarball.
Remove the prototype, it's now in its own repo.
Switch from 'bundle update' to 'bundle' because it's safer.

We want to increase the likelihood of success on that first run through. 'bundle' will use the same gems that have passed on CI, so they should work for the user too.
TODO is old and not really relevant anymore.
Update changelog.
Don't need the cleanup script anymore.