Simplify bundler config so "bundle install" does the right thing by default

This also makes it so that when we `ONBUILD RUN bundle install` it doesn't create `.bundle/config` in our precious, pristine application directory, and makes all the bundler-installed Gems show up in `gem list`.

I'm also sneaking in a change to the `onbuild` variants that makes `bundle install` just fail if you've updated your `Gemfile` but not your `Gemfile.lock`, since that bit me pretty hard while testing this (ie, `bundle install` was silently updating `Gemfile.lock`, which changes were then clobbered by the `COPY . /usr/src/app` and the application wouldn't run).  I _think_ this might actually be the real issue in https://github.com/docker-library/rails/issues/11, too.