Do not include bootstrap dependencies.

@lunaryorn The issue https://github.com/cask/cask/pull/85 was about bootstrap dependencies not being reset on bootstrap. You did this commit to fix it https://github.com/cask/cask/commit/3e37108cb9e7059c0666e98d7ca342149e460396.

Not sure why, but it looks like the bug is back. The scenario below should not fail, but does, because the bootstrap `s` path is included in `load-path`.