`bundle install` without a lockfile should install the latest bundler and use that to install dependencies

[deivid-rodriguez]

I think that would get the best behaviour available for bundle install, encourage upgrading and reduce bug reports for things that have already been fixed.

[deivid-rodriguez]

Similarly bundle update could update bundler and then use that to update the rest of the dependencies.

[deivid-rodriguez]

And relatedly, bundle install with a lockfile should install the version in the BUNDLED WITH section of the lockfile, and then use that to install dependencies. This is what #4076 is trying to implement.