Option to auto-bundle before commands if `bundle check` is failing

This PR addresses bundler/bundler-features#9 and is an update to @smashwilson's PR #2810.

I rebased to the latest version of Bundler in master and updated the code to fix conflicts. As per @indirect's instruction, also replaced the use of an env variable with `bundle config auto_install 1`.

Let me know if there needs to be any changes made.