Require `bundler/setup` if ENV var `BUNDLE_GEMFILE` set

This is to make sure that a bundled environment is always loaded
regardless of how this is called.

@kellyredding - It's only the latest commit, the other is from the #54. Do you think this is necessary or a good practice? I haven't noticed any differences with or without it, but logically, it seems something like this is needed if you aren't calling it with `bundle exec`. For example, the hot restart doesn't use `bundle exec` and I'm not sure there's a good way to add it without locking you into having bundler. Thoughts?