implement all_platforms options for cache/package

so that a single cache directory can be used with multiple ruby versions

we use bundle package to build a package that we then push out to all servers, but when slow-rolling a new version of ruby (i.e. upgrading from 1.9 to 2.1), the cache directory will only have the gems pertinent to the version of ruby used to package. by adding --all-platforms, the cache directory will have them all, and a subsequent bundle install --deployment --local will work correctly, regardless of if the server is running 1.9 or 2.1.