Use kiex to install and manage multiple Elixir versions. This uses functions described in Find up with Alternates that should also be in your direnvrc.

Put use_kiex in your ~/.config/direnv/direnvrc:

use_kiex()
{
  has kiex || return

  local version
  version="${1}"

  [[ "${version}" == --auto ]] && version="$(read_version_file .elixir-version)"
  [[ -z "${version}" ]] && return

  local activate

  activate="${KIEX_HOME:-${HOME}/.kiex}"/elixirs/"$(
    kiex list |
      ruby -e "puts ARGF.read.scan(/elixir-${version}.*/).last"
  )".env

  [[ -f "${activate}" ]] && source "${activate}"
}

This is a conservative fuzzy use_ function: it does not fail if kiex is not installed or if the Elixir version is not actually installed or usable. It does fuzzy matching preferring newer versions over earlier versions so that use kiex 1.7 will find Elixir 1.7.4 if that is the latest 1.7 version installed.

This can also read a .elixir-version file containing a version specification so that your .envrc may specify use kiex --auto.