Warn about excluding a previously excluded group from a local config file

This was the second time I was tripped by the behavior "If you run bundle install later, without any flag, bundler will remember that you last called bundle install --without production, and use that flag again."

For the principle of least surprise, I added a warning that will be shown when the without option is active, because of a local configuration file and not because of a command line parameter.

Test and code in the commit (I ran the rspec tests on 1.8.7 with a fairly old set of gems; I did not test it in travis).