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
Merge pull request #212 from heroku/schneems/parallel-bundle-wrapper-fix

Fix parallel bundler tests