[close #205] Check for Rakefile before rake runs

If no Rakefile (or variant) is present we should not attempt to detect rake tasks.
Fix parallel bundler tests

- Always use a tmpdir to download bundler.
- Remove un-used methods `Ruby.bundle` and `Ruby.bundler_path`.
- Re-name `install_language_pack_gems` to `install_bundler_in_app` since that's all it ever does
- Copy bundler into app path instead of re-downloading
update heroku hatchet 1.1.9