Bump Elixir to 1.8.1-R21 (Nix version)
Bump Elixir to 1.8.1-R21 (Nix version)

Uses new `verman` ability to install OTP-version-specific versions of
Elixir.

benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles