Without this config update bundler will raise errors, it's a weird default they use (see this issue):
bundle config disable_local_branch_check true
The add the config for the gem to make bundler look to the path you specify rather than where it has downloaded the gem too, as source:
bundle config local.YOUR_GEM_NAME YOUR_GEMS_PATH
Now you can bundle from your local source without commenting out the production version of the gem. No more dirty always staged changes for development only settings.