Warn if bundler version older than in lockfile

https://github.com/bundler/bundler-features/issues/17

indirect: This probably requires adding the version of Bundler that generated the lockfile to the lockfile. Since that is a change to the format of the lockfile, this change can't happen until 2.0. That said, I think that this is really important to do for 2.0, especially so that we can ask people to upgrade their old Bundler versions.

Goes on the **2-0-dev branch**