add rbenv-default-gems formula

Everything appears to work correctly as long as you put the `default-gems` file in the proper location. The [README](https://github.com/sstephenson/rbenv-default-gems/blob/master/README.md) states:

    rbenv-default-gems automatically installs the gems listed in the ~/.rbenv/default-gems 
    file every time you successfully install a new version of Ruby with rbenv install.

In actuality, when installed via homebrew, the `default-gems` file must be placed in `$RBENV_ROOT` (which defaults to `/usr/local/opt/rbenv` instead of in `~/.rbenv` to be picked up correctly. It's actually makes sense but since it might not be obvious to people, maybe this needs to be added as a caveat on the formula?

In the meantime, I've opened an issue sstephenson/rbenv-default-gems#1 to see if the README could be clarified.