Removed the rbenv-default-gems Homebrew install.

- [Ruby Gem Setup](https://github.com/bkuhlmann/ruby_gem_setup) is
  used instead.
Removed the rbenv-gem-rehash Homebrew install.

- Conflicts with Bundler parallel installs.
- There is an open issue and PR on the project but it doesn't look
  like it's going to be resolved anytime soon.
Removed the rbenv-binstubs Homebrew install.

- No longer used, use the "be*" commands found in the
  [Dotfiles](https://github.com/bkuhlmann/dotfiles) project instead.