If a gem is specified in the Gemfile (or resolved as a transitive
dependency), it's always resolved from remote/installed sources. Default
gems are only used as a fallback for gems not included in the bundle.
I believe this leads to more consistent behavior and more portable apps,
since all gems will be installed to the configured bundle path,
regardless of whether they are default gems or not.
https://github.com/rubygems/rubygems/commit/091b4fcf2b