Bundler flag --force-native as a possible workaround fix for #646 and #635

What do you think about adding this flag to allow deployments to relax the freeze restriction in the case where the Gemfile.lock came from a different platform?

The problem I *think* I'm fixing is that:

bundle --deployment

sets "freeze", which prevents any changes to the Gemfile.lock.  And this prevents gem updates that are needed if you move platforms.

I considered proposing a --no-freeze option, but I only want to --no-freeze in cases where it's necessary (aka cases where the Gemfile.lock was created on a different platform)

I'm not married to the name "force-native" or even implementation... just REALLY interested in the behavior.

thanks!